CHARACTERIZATION OF FLAX SHIVES AND FACTORS AFFECTING THE
QUALITY OF FUEL PELLETS FROM FLAX SHIVES
A Thesis Submitted to the College of
Graduate Studies and Research
In Partial Fulfillment of the Requirements
For the Degree of Master of Science
In the Department of Agricultural and
Bioresource Engineering
University of Saskatchewan
Saskatoon
By
Bayartogtokh Rentsen
© Copyright Bayartogtokh Rentsen, March 2010. All rights reserved.
i
COPYRIGHT
The author has agreed that the Libraries of this University may make this
thesis freely available for inspection. In this thesis, in partial fulfillment of the
requirements for a Postgraduate degree from the University of Saskatchewan, I
further agree that permission for copying of this thesis in any manner, in whole or in
part, for scholarly purposes may be granted by the professor or professors who
supervised my thesis work or, in their absence, by the Head of the Department or the
Dean of the College in which my thesis work was done. Any copying or publication
or use of this thesis or parts thereof for financial gain will not be allowed without my
written approval. It is also understood that due recognition will be given to the author
and to the University of Saskatchewan in any scholarly use which may be made of
any material in my thesis.
Requests for permission to copy or to make other use of material in this thesis in
whole or in part should be addressed to:
Department Head of Agricultural and Bioresource Engineering
University of Saskatchewan
Saskatoon, Saskatchewan S7N 5A9
Canada
ii
ABSTRACT
Flax shives are a source of abundant biomass from renewable sources. They
are considered to be environmentally benign and have a high-energy content for
heating and generation of electricity, but only after being processed into pellets.
Pelleting of the shives was done by using the single-pelleter and pilot-scale mill. The
effect of grinding with screens of 2.4, 3.2, and 6.4 mm on unit density and durability
was conducted with a completely randomized design using shives from Biofibre
Industries Inc., Canora, SK. The central composite face-centered design with 3 levels
of lower grade canola meal used as a binder (18, 21, and 24%), moisture content (8,
11, and 14% (w.b)), and hammer mill screen size (3.2, 4.8, and 6.4 mm) was used to
determine the effects of these three factors on the properties of fuel pellets made
from shives obtained from Biolin Research Inc., Saskatoon, SK.
The initial moisture content of coarse flax shives from both sources was
about 10.5% wet basis (w.b.). The moisture content of flax shive grinds ranged from
9.6 to 10.5% (w.b.) after grinding, using the smaller screens for the Biofibre material,
while the moisture content ranged from 7.9 to 8.6% (w.b.) for shives from Biolin.
Also, smaller screen size reduced the geometric mean particle size for shives from
both sources. The use of the smaller hammer mill screen resulted in an increase in
both bulk and particle density of shives. There was a decrease in coefficient of the
internal friction of shives from 0.20 to 0.14 and an increase in a cohesion of shives
from 2.18 to 3.83 kPa when the screen size decreased from 6.4 to 3.2 mm. The flax
shives contained cellulose (53.27%), hemicelluloses (13.62%), and lignin (20.53%)
at a moisture content of 7.9% (w.b). Specific heat capacity of flax shives changed
from 1.5 to 2.7 kJ/ (kg °C) when the moisture content was increased from 8 to14%
iii
(w.b.) and temperature from 15 to 80°C. The shives had the combustion energy of
17.67 MJ/kg at a moisture content of 8.1% (w.b.).
The smallest screen size (2.4 mm) resulted in the highest unit density (1010
kg/m3) and the highest durability (88%) in the pellets produced by the single-
pelleting equipment. The change in length of pellets produced by the pilot-scale mill
increased as canola meal increased from 18 to 24% at the highest moisture content
(%). The pellets were more stable at the highest moisture content when the lowest
canola meal used. The addition of 18% canola meal and grinds from a screen size of
6.4 mm produced the highest unit density in the pellets at all moisture levels. The
highest bulk density (682 kg/m3) was obtained from shive mixtures with 18% canola
meal and a moisture content of 8%. The highest hardness and durability were found
for the shive pellets that were produced with 18% canola meal at a moisture content
of 14% (w.b). Pellets that were produced at a moisture content of 14% (w.b) resulted
in the lowest percentage of moisture absorption.
The inclusion of the canola meal in the shive mixture resulted in an increase
in the combustion energy of the pellets because of the fat content in the binder. The
two levels of canola meal for shive pellets had essentially the same level of
emissions. However, there were significant differences between shive pellets and
commercial wood pellets in the level of the emissions. Lower amounts of methane
(1.29 ppm) and oxygen (164.3 ppt) were found for flax shive pellets than of methane
(1.63 ppm) and oxygen (176.6 ppt) in commercial wood pellets.
In short, pelleting of flax shives into fuel pellets improved the handling
characteristics, increased bulk density and energy content. Fuel pellets made from
flax shives had less emission of methane and oxygen from combustion when
compared to commercial wood pellets.
iv
ACKNOWLEDGEMENTS
Firstly, I would like to thank God, the most gracious and the most merciful,
for giving me the opportunities to learn and study through my life and this research.
I would like to express my deepest appreciation to my graduate supervisor,
Dr. Satyanarayan Panigrahi, for his generous support and continuous guidance and
advices, as well as his friendship, patience, and understanding in every situation
through this work. A heartfelt thank you goes out to each member of my graduate
advisory committee, Dr. Charles Maule, Dr. Lope Tabil, Jr and Dr. Oon-Doo Baik
for their supervision, motivation and valuable scientific advices throughout my
thesis. A special thank you goes to my external examiner, Dr. Catherine Niu.
I also would like to extend my sincere thanks to the technical staff and all my
colleagues at the Department of Agricultural and Bioresource Engineering. Thank
you to the Sun West Food Laboratory and Department of Soil Science at the U of S
for their assistance with the chemical analysis of material and emission analysis of
burned gas samples. Also, I would like to say many thanks to my friends, Brain &
Jan Townsend, Jim & Ruth Massey, David & Barbara Spurr, Elmer & Anne
Guenther, Harvey Sauder, Dr. Xue Li, Dr. Mohammad Izadifar, Dr. Lal Kushwaha,
Dr. Paul Stevens, and Barney Habicht for donating their time and support throughout
my life and thesis work in a variety of situations. Also, thanks to my most sincere
gratitude to my wife, Oyuntamir Delgerjav and son, Sumiyabazar Bayartogtokh.
Without their support, encouragement and patience, it would not have been possible
to complete this thesis.
Finally, many thanks go to the Mongolian State Training Fund, ABIP-SBQQ,
FOBI and NSERC for their grant and financial assistance throughout my thesis work.
v
DEDICATION
This thesis is dedicated to my dear friend Grace Kathryn Sauder (June 5, 1934-
October 21, 2009). Grace was encouraging and supporting me to continue my
education.
vi
TABLE OF CONTENTS
COPYRIGHT ........................................................................................................... i
ABSTRACT ............................................................................................................ ii
ACKNOWLEDGEMENTS .....................................................................................iv
DEDICATION ......................................................................................................... v
TABLE OF CONTENTS .........................................................................................vi
LIST OF TABLES .................................................................................................... x
LIST OF FIGURES .................................................................................................xi
NOMENCLATURE ............................................................................................. xiii
1. INTRODUCTION ................................................................................................ 1
1.1 Objectives ...................................................................................................4
1.2 Organization of the Thesis ..........................................................................4
2. LITERATURE REVIEW ...................................................................................... 6
2.1 Raw Material Properties Affecting Biomass Densification ..........................6
2.1.1 Particle size and particle size distribution ....................................7
2.1.2 Bulk and particle density .............................................................8
2.1.3 Moisture content .........................................................................9
2.1.4 Chemical composition ............................................................... 11
2.1.5 Angle of internal friction and cohesion ...................................... 12
2.2 Process Variables Influencing Biomass Densification ............................... 13
2.2.1 Pressure .................................................................................... 13
2.2.2 Preheating temperature .............................................................. 14
2.2.3 Binding of particles ................................................................... 15
2.2.4 Steam conditioning ................................................................... 18
2.2.5 Die geometry and die speed ...................................................... 19
vii
2.3 Physical Quality of Densified Products ..................................................... 20
2.3.1 Dimensional stability and densities ........................................... 21
2.3.2 Tensile strength ......................................................................... 22
2.3.3 Hardness ................................................................................... 23
2.3.4 Durability .................................................................................. 24
2.3.5 Moisture absorption .................................................................. 25
2.4 Specific Energy Requirement and Economics ........................................... 26
2.4.1 Specific energy required to grind biomass ................................. 26
2.4.2 Specific energy required to form pellets .................................... 28
2.4.3 Economics of pelleting .............................................................. 29
2.5 Emissions.................................................................................................. 29
2.6 Summary .................................................................................................. 30
3. MATERIALS AND EXPERIMENTAL METHODS .......................................... 33
3.1 Materials and Characterization .................................................................. 33
3.1.1 Particle size and size distribution............................................... 36
3.1.2 Bulk and particle density ........................................................... 37
3.1.3 Moisture content and conditioning ............................................ 39
3.1.4 Chemical components ............................................................... 40
3.1.5 Angle of internal friction and cohesion ...................................... 41
3.1.6 Specific heat capacity ................................................................ 42
3.1.7 Combustion energy ................................................................... 43
3.2 Material Processing Equipment and Procedures ........................................ 46
3.2.1 Pelleting with the single pelleter ................................................ 46
3.3.2 Pelleting with the pilot-scale pellet mill ..................................... 48
3.3 Quality Properties of Pellets ...................................................................... 52
3.3.1 Dimensional stability ................................................................ 52
viii
3.3.2 Densities of pellets .................................................................... 53
3.3.3 Hardness ................................................................................... 53
3.3.4 Durability .................................................................................. 54
3.3.5 Moisture absorption .................................................................. 55
3.3.6 Combustion energy ................................................................... 56
3.4 Emission Measurement ............................................................................. 56
3.5 Statistical Analysis .................................................................................... 58
4. RESULTS AND DISCUSSION .......................................................................... 60
4.1 Physical Properties of Flax Shives and Canola Meal ................................. 60
4.1.1 Moisture content ....................................................................... 61
4.1.2 Particle size ............................................................................... 61
4.1.3 Particle size distribution ............................................................ 63
4.1.4 Bulk and particle density ........................................................... 65
4.1.5 Frictional behaviour of biomass grinds ...................................... 67
4.2 Chemical and Thermal Properties.............................................................. 68
4.2.1 Chemical composition ............................................................... 68
4.2.2 Specific heat capacity of flax shives .......................................... 69
4.2.3 Combustion energy ................................................................... 71
4.3 Effect of Flax Shive Particle Size on Pellet Density and Durability ........... 71
4.3.1 Unit density of pellets made with single-pelleter ....................... 72
4.3.2 Pellet durability ......................................................................... 73
4.4 Effect of Particle Size, Moisture Content, and Canola Meal Levels on the Physical Properties of Flax Shive Pellets Produced in the Pilot-scale Pellet Mill .......................................................................................................... 73
4.4.1 Dimensional stability ................................................................ 74
4.4.2 Unit density of pellets ............................................................... 80
4.4.3 Bulk and particle density ........................................................... 82
ix
4.4.4 Hardness ................................................................................... 87
4.4.5 Durability .................................................................................. 89
4.4.6 Moisture absorption .................................................................. 90
4.5 Combustion Energy and Emissions ........................................................... 92
4.5.1 Combustion energy ................................................................... 92
4.5.2 Emissions .................................................................................. 92
4.6 Summary .................................................................................................. 94
5. CONCLUSION .................................................................................................. 98
6. RECOMMENDATIONS FOR FUTURE STUDIES ......................................... 102
REFERENCES ..................................................................................................... 104
APPENDIX A ...................................................................................................... 112
APPENDIX B ....................................................................................................... 114
x
LIST OF TABLES
Table 3.1 Central Composite Face-Centered Design (CCFD) ................................. 51
Table 3. 2 Evaluation tests performed and equipment used. .................................... 52
Table 4. 1 Measured physical properties of flax shives and canola meal. ................ 60
Table 4. 2 Moisture content of flax shives. ............................................................. 61
Table 4. 3 Geometric mean particle size (dgw) and standard deviation (Sgw) of shives. ............................................................................................................................... 62
Table 4. 4 Means of bulk and particle density and porosity of ground flax shives from Biofibre Industries Inc.. ................................................................. 65
Table 4. 5 Means of bulk and particle density and porosity of ground flax shives from Biolin Research Inc.. ..................................................................... 66
Table 4. 6 Angle of internal friction and cohesion of shives. .................................. 67
Table 4. 7 Chemical composition of flax shives and canola meal (percent dry matter basis). .................................................................................................... 68
Table 4. 8 Specific heat capacity of flax shives at six levels of temperature and three levels of moisture content. ..................................................................... 70
Table 4. 9 Unit density and durability of pellets at three levels of grinding. ............ 72
Table 4. 10 Percent change in length, diameter, and ratio of length/diameter of pellets during storage with standard error in parentheses. ..................... 74
Table 4. 11 Least squares means of unit density of shive pellets. ............................ 80
Table 4. 12 Bulk and particle density and porosity of flax shive pellets. ................. 82
Table 4. 13 Least squares means and standard error of hardness of shive pellets. ... 88
Table 4. 14 Least squares means and standard error of durability of shive pellets. .. 89
Table 4. 15 Least squares means and standard error of moisture absorption of shive pellets. ................................................................................................. 91
Table 4. 16 Emissions from flax shive pellets and wood pellets. ............................. 93
xi
LIST OF FIGURES
Figure 3. 1 Materials: (a) coarse flax shives, (b) canola meal used as a binder. ....... 34
Figure 3. 2 Material characterization of flax shives. ............................................... 34
Figure 3. 3 Wykeham Farrance shear box apparatus. .............................................. 42
Figure 3. 4 Differential Scanning Calorimetry, DCS 111 for specific heat capacity measurement of samples. ..................................................................... 42
Figure 3. 5 Bomb calorimeter set up. ...................................................................... 44
Figure 3. 6 Bomb calorimeter; (a) Gas cylinder and oxygen bomb, (b) Bomb
calorimeter in water container with ignition switch, and thermometer .. 45
Figure 3. 7 Single pellet equipment. ....................................................................... 46
Figure 3. 8 Instron® model 1011 testing machine with attached single pelleter. ..... 47
Figure 3. 9 California Pellet Mill. ........................................................................... 48
Figure 3. 10 Processing scheme of flax shives for fuel pellets. ............................... 49
Figure 3. 11 Die and roller assembly; (a) Die, (b) Roller. ....................................... 49
Figure 3. 12 Hardness test assembly. ...................................................................... 54
Figure 3. 13 Grain burning stove used to measure emissions. ................................. 57
Figure 3. 14 Some components of the grain burning stove; (a) Firebox, (b) Outlet where gas samples were collected, (c) Fuel control knob, (d) Room temperature control knob. ................................................................... 58
Figure 4. 1 Particle size distribution of shives from Biofibre Industries Inc. at various screen sizes. ............................................................................. 64
Figure 4. 2 Particle size distribution of shives from Biolin Research Inc. at various screen sizes. ......................................................................................... 65
Figure 4. 3 Graph of estimated specific heat capacity of flax shives with respect to ... moisture content and temperature….…………………………………...70
Figure 4. 4 Flax shive pellets produced by a single-pelleter. ................................... 72
Figure 4. 5 Flax shive pellets produced using a pilot-scale pellet mill. .................... 73
xii
Figure 4. 6 Change in pellet length at various levels of screen size and canola meal content. ................................................................................................ 75
Figure 4. 7 Change in pellet length at various screen sizes and moisture contents. .. 76
Figure 4. 8 Change in pellet length at various canola meal and moisture content. ... 77
Figure 4. 9 Lateral expansions of pellets containing three level canola meal produced at three level moisture content ............................................... 78
Figure 4. 10 Length to diameter of the flax shive pellets at various screen size and canola meal content. .......................................................................... .79
Figure 4. 11 Change in length to diameter ratio of the flax shives pellets at various ... screen sizes and moisture contents……………………………………79
Figure 4. 12 Unit density of the pellets produced at various levels of moisture and canola meal contents ......................................................................... 81
Figure 4. 13 Bulk density of flax shive pellets at various moisture and canola meal .. content………………………………………………………………...83
Figure 4. 14 Bulk density of flax shive pellets at various levels of screen size and canola meal. ........................................................................................ 83
Figure 4. 15 Effect of screen size and moisture content of bulk density of pellets. .. 84
Figure 4. 16 Particle density of pellets at levels of screen size and canola meal. ..... 85
Figure 4. 17 Particle density of pellets at three levels of screen size and moisture content. ............................................................................................... 86
Figure 4. 18 Particle density of pellets at three levels of canola meal and moisture content. ............................................................................................... 87
Figure 4. 19 Effect of canola meal and moisture content on pellet hardness. ........... 89
Figure 4. 20 Effect of moisture and canola meal content on pellet durability. ......... 90
Figure 4. 21 Effect of screen size and moisture content on moisture absorption of flax shive pellets. ................................................................................ 91
xiii
NOMENCLATURE
Cc= cohesion (kPa)
Cp = specific heat capacity (J/(kg°C))
d.b. = dry basis (%)
df = degree of freedom
dgw = geometric mean particle size (mm)
dH/dt = heat flow rate (J/s)
e = correction for the heat of firing fuse wire (J)
H = energy content of the sample (J/kg).
Hg = energy content of standard benzoic acid (J/kg)
l/d = length-to-diameter ratio
m = mass (g)
ma = mass of benzoic acid (g)
mi = initial mass of the sample (g)
mw = mass of water added to the sample (g)
Mwf = final desired moisture content (wet basis, w.b.) of the sample
Mwi = initial moisture content (w.b.) of the sample
MS = Mean square
n = number of replicates
PDI = pellet durability of index
P1 = pressure reading after pressurizing the reference volume (kPa)
P2 = pressure reading after including volume of the cell (kPa)
R2 = coefficient of determination
S.E. = standard error
xiv
SEE = standard error of estimate
Sgw = geometric standard deviation of particle size (µm)
t = time (s)
t = corrected temperature rise (°C)
T = sample temperature (°C)
V = volume of compacted sample at pressure P (cm3
)
V0 = volume of sample at zero pressure (cm3
)
VC = volume of cylinder (cm3
)
Vcell = volume of the cell (cm3
)
VR = reference volume for the large cell (cm3
)
Vs = volume of solid (cm3
)
W = weight of sample (g)
W = energy equivalent of calorimeter (J/K)
w.b. = wet basis (%)
Wd = moisture content (% d.b.)
Ww = moisture content (% w.b.)
α = distance of axial point from the center
ε = porosity (%)
µ = coefficient of friction
ρb = initial bulk density (kg/m3)
σ = normal stress (Pa)
ρt = particle density (kg/m3)
τ = shear stress (Pa)
φi = angle of internal friction (degree)
1
1. INTRODUCTION
In recent times, many of man’s practices have resulted in negative effects to
the environment. In addition, the burning of fossil fuels has enormous negative
effects on human health. The fossil fuels energy source including coal, oil, and
natural gas is used worldwide for heating and electricity generation. These sources of
energy, which have been used for a long time, are non-renewable and eventually
these will be depleted. With the cost increase to produce the energy from fossil fuels,
there has been a corresponding increase in interest in alternate sources for heating,
such as solid fuels. As a result, a global effort has been started to generate renewable
sources of energy which can meet increasing energy needs.
In response to these factors, researchers and engineers are developing
methods of using fuels from biomass. Biomass is any biological material originated
from living organisms including wood waste, forest residues such as straw, saw dust,
shavings, corn stover and switchgrass. According to Demirbas (2004), biomass
contributes the balance of the atmospheric carbon dioxide in the air and does not add
to the greenhouse effect to the same degree as the use of fossil fuels.
Various types of biomass have been studied extensively as a source of biofuel
by densification. The first U.S. patent for densification was issued in 1880 (Reed and
Bryant 1978), and the method was commonly used for producing animal feed.
Densification can be done by extrusion, pelletizing, and briquetting (Li and Lui
2000; Pietsch 2002). Densification of biomass in the form of cubes, briquettes, and
pellets is achieved by applying a mechanical force to particles causing them to bond
2
together and is affected by both raw material properties and process variables. Raw
material properties such as particle size, size distribution, bulk and particle density,
and chemical components were studied on the densification. In the densification
process, grinding creates inter-particle bonding, as well as, well-defined shapes and
sizes of pellets, briquettes, and cubes (Kaliyan and Morey 2006).
It is important that densified products meet the quality criteria that included
higher bulk density, higher hardness, higher durability, higher energy content and
lower gase emissions. Therefore, the properties of the end products made from
various biomass materials were also studied extensively. The solid fuel made from
biomass in the form of pellets must have uniform shape and size; high bulk density;
and high hardness and durability. Pellets must also have resistance to moisture
adsorption and have high energy content, as well as low emissions.
Flax straw is one example of an abundant biomass that has potential as a
biomass fuel. In Canada, many farmers grow oilseed flax for the production of the
oil. After harvesting the seeds, the flax straw, composed of fiber and shive, is left in
the field. Since flax straw biologically degrades slowly in the field, farmers often
burn it in order to get rid of this residue. The fibers have been used commercially in
the traditional textile sector, as filler in plastic composite materials, as insulation
bats, and for other industrial purposes. However, only a small percentage of the flax
straw produced in Canada is processed for the extraction of fiber. Schweitzer-
Mauduit Canada is the only large processor of oilseed flax fiber in Canada producing
between 27,000 and 40,000 tonnes of roughly cleaned fiber per year (Ulrich 2008).
After the fibre has been removed from the straw, the residue is known as shives
which are non-fibrous by-products consisting of a mixture of particles with various
chemical and physical structures. Oilseed straw contains about 78% shives with an
3
average price of approximately $2 per tonne (Ulrich 2008). Shives consist of both
cellulose and hemicelluloses, and lignin (Shaw and Tabil 2005) that acts as a binding
agent when processing. They also can be used for animal bedding, particle board,
horticulture applications, filler in bio-composites, and biofuel markets. In addition, it
was stated on the website of Irish Linen (2002) that 2 kg of shives have the same
heating value as 1 litre of fuel oil.
The above statements indicate that flax shives have potential value and could
be utilized as biofuels which add fewer pollutants to the environment, thereby
reducing some of the problems of global warming brought about by greenhouse gas
emissions. However, there are a number of problems related to using flax shives as a
raw material in the production of biofuel pellets. One of the major problems of using
shives as a solid biofuel is their low density. It is difficult to handle shives in their
current form because they require large spaces to store and transport. Without further
compression, storage and transportation costs would be too high to use this material
economically for fuel. Furthermore, bulky and dusty shives can be a fire hazard
during handling and storage.
The solution of the above problems is to densify them in the form of pellets.
Pelleting brings particles together applying the mechanical forces to the particles
during processing. Therefore, this improves the handling characteristics, reduces the
storage requirement, increases the energy content, and decreases the emissions. Final
quality of the pellets can be affected by both the raw material and processing factors.
The densified products will contribute to value-added processing and to total
utilization of flax crops giving additional environmental benefits.
4
1.1 Objectives
The overall objective of this research was to study the criteria for
manufacturing biofuel pellets from flax shives, a residue or co-product of oilseed flax
straw processing, using a single pelleting unit and a pilot-scale pellet mill. The
specific objectives were:
1. to characterize flax shives in terms of physical, chemical, and thermal
properties including particle size and distribution, densities, heat of combustion,
specific heat capacity, and composition;
2. to investigate the effects of particle size, moisture content, and the addition
of canola meal on the physical properties of flax shive pellets; and
3. to measure the heat of combustion of the pellets and to analyze the gas
emissions during combustion of shive pellets.
1.2 Organization of the Thesis
This research work is presented in six chapters namely: Introduction,
Literature Review, Materials and Experimental Methods, Results and Discussion,
Conclusion and Recommendations. In the Literature Review, both the raw material
properties and processing parameters affecting biomass densification are reviewed.
In addition, the physical properties of the compacts (i.e. pellets and briquettes) along
with the specific energy consumption required to grind feedstocks in the formation of
fuel products are presented. The chapter ends with a discussion of emissions from
biomass fuel pellets after combustion, and a summary of the literature review. In the
Materials and Experimental Methods chapter, the equipment and methodology used
for measuring the physical, chemical, and thermal properties of the materials and fuel
pellets are described. The Results and Discussion section presents and discusses the
5
experimental values obtained for the physical properties, chemical composition and
thermal properties of the raw material. This chapter also presents the effects that the
properties have on the dimensional stability, densities, hardness, durability, and
moisture absorption of fuel pellets, a statistical analysis used to find differences and
significance among the different treatments used to manufacture pellets. Finally, the
Conclusion is presented followed by the Recommendations which enumerate the
issues to be addressed in future studies.
6
2. LITERATURE REVIEW
This chapter reviews how material properties and processing parameters
affect mechanical properties of pellets and briquettes made from a variety of raw
materials. In addition, properties of densified products made from a variety of
biomass feedstock are reviewed. The energy content of biomass fuel product and the
specific energy required to grind the biomass and produce the products along with
the emissions from combustion are also reviewed in this chapter.
2.1 Raw Material Properties Affecting Biomass Densification
Densification of material such as animal feed and biofuel from biomass
including sawdust, straws, wood chip, shavings, corn stover, switchgrass, and many
others has been studied extensively (Haussman 1975; Wamukonya and Jenkins 1995;
Tabil 1996; Tabil and Sokhansanj 1996; Thomas et al. 1996; Mani et al. 2004a; Rhen
et al. 2005, Mani et al. 2006b; Mozammel et al. 2006; Kaliyan and Morey 2006;
Shaw 2008). However, no research work on densification of flax shives was found in
the literature. Raw waste material such as the flax shives shown is a heterogeneous
mixture with a range of particle sizes and a complex mixture of chemical
components and physical structures. Raw material properties such as particle size,
particle size distribution, unit density, particle density, bulk density, and moisture
content affecting densification are of primary interest in this study, since the quality
of final densified products depends on the raw material variables. Therefore, this
7
section summarizes previous studies of how the raw material properties listed above
along with the angle of internal friction affect the densification and properties of
products made out of a variety of raw materials.
2.1.1 Particle size and particle size distribution
Raw materials must be reduced in particle size before they are processed into
fuel pellets by densification. Size reduction, one of the main operations in biofuel
pellet production, can be done using a variety of grinders. The grinds produced have
a wide range of particle sizes which can be controlled by the screen size of the
grinder. Mani et al. (2004a) stated that particle size and particle size distribution of
biomass materials have a significant influence on the binding characteristics of
densification and are also important considerations in the design of pneumatic
conveyors and cyclones. Mechanical properties of the materials and final product
properties are affected by the particle size and its distribution. Therefore, when
reducing the particle size of raw materials using a hammer mill, it is important to
ensure that the screen size used results in higher bulk and particle densities. The
geometric mean particle size is a more realistic expression of the mean size of the
particles since the size of the biomass particles has a logarithmic distribution. Mani
and co-workers (2006b) reported that wheat and barley straw grinds had a geometric
standard mean particle size of 0.64 and 0.69 mm respectively when ground using a
hammer mill with a screen size of 3.2 mm. They also observed that reducing the
geometric mean particle size from 0.64 to 0.28 mm of wheat straw grinds resulted in
an increase of bulk density from 97 to 121 kg/m3 and particle density from 1030 to
1340 kg/m3. Shaw and Tabil (2006) found that the geometric average of the particle
size of flax shives was 0.64 mm using a hammer screen of 6.4 mm. For corn stover,
8
the geometric mean particle size was 0.68 mm when using a 6.4 mm screen size
(Mani et al. 2004b). Mani et al. (2004b) also reported that the particle density of corn
stover grind increased with a decrease in the geometric mean particle size of the
grind. Decreasing the particle size of corn stover from 0.80 to 0.66 mm increased the
relaxed density by 5 to 10%, and the briquette durability from 62 to 75% at 150 MPa
pressure (Kaliyan and Morey 2006). This occurs since there is more particle surface
area available for bonding at a smaller particle size than at a larger particle size.
The particle size distribution of raw materials is important since it affects raw
material properties including moisture content, bulk and particle densities, which in
turn affect the densification process, and final product properties such as density,
hardness, and durability. Mani et al. (2004a) found that grinding wheat straw and
barley straw using a hammer mill with a screen size of 3.2 mm gave a wide range for
particle size distribution, and particles distributed in this range were suitable for
compaction (pelleting or briquetting) process. Tabil and Sokhansanj (1997)
emphasized that the alfalfa grind particles with a size below 0.4 mm are considered
fine and are easily compressed into pellets.
2.1.2 Bulk and particle density
The bulk and particle densities of the raw materials and grinds are functions
of the particle size and moisture content. Shaw and Tabil (2006) found that the bulk
density of flax shives using a hammer mill with a 6.4 mm screen was 107.99 kg/m3.
Mani et al. (2004b) found that ground corn stover had a bulk density ranging from
42.25 kg/m3 for coarse material (tub ground chop) to 130 kg/m3 for finer material
using a hammer mill screen size of 3.2 mm. These two sets of results indicate that
flax shives and corn stover have similar bulk densities. Mozammel et al. (2006)
9
found that bulk density of small size wood chips increased from 180 to 314 kg/m3
when the moisture content increased from 10.7 to 55.7% wet basis (w.b.). The length
of the small size chips ranged from 10 to 25 mm. Also, they reported that the bulk
density of barley straw ranged from 24 to 54 kg/m3 as the average particle size
decreased from 50 to 6 mm respectively at a moisture content of 8.45% (w.b.). Tabil
and Sokhansanj (1997) reported that the bulk density of the low, medium, and high
quality alfalfa grinds at a moisture content of 5.3, 5.4, and 6.2% (w.b.) was 238, 263,
and 236 kg/m3, respectively. Shaw (2008) studied the bulk and particle densities of
selected feedstock, namely, poplar, pretreated poplar, and wheat straw at two
moisture levels (9 and 15% w.b.) using screen sizes of 0.8 and 3.2 mm. Both bulk
and particle densities of the feedstock decreased with an increase in moisture and
screen size.
2.1.3 Moisture content
The knowledge of the effect of moisture content on the properties of the raw
material is important for determining steps in the processing of raw materials.
Moisture content can affect the specific energy required to compact biomass
materials as well as properties such as densities, angle of friction, specific heat
capacity, force-deformation characteristics, and thermal conductivity. The initial
moisture content of the grinds also has a significant effect on the densification
process and final product properties. Mani et al. (2004c) studied the compaction
mechanism of four biomass grinds wheat and barley straws, corn stover, and
switchgrass at different applied forces, moisture contents, and particle size. They
concluded that at low moisture levels (5 and 10% w.b.) corn stover produced more
stable and durable briquettes with high-density than at a higher moisture level (15%
10
w.b.). Orth and Lowe (1977) found that moisture content of hay below about 12%
(w.b) did not form stable wafers in a continuous extrusion system without
temperature control. However, increasing the moisture content from 12 to 16% (w.b.)
resulted in higher durability of the wafers, while the maximum density of the wafers
was reached at 14% (w.b.). They also reported that both density and durability
decreased with a further increase in the moisture content. Moisture content has an
effect on the specific energy for compacting ground materials. Mani et al. (2006a)
found that the moisture content of corn stover greatly affects the magnitude of
compression and extrusion energy required to produce briquettes. They found that by
increasing the moisture content (10-15% w.b.) less total energy was consumed. Carre
et al. (1987) stated that very low moisture content in wood residues made it difficult
to produce briquettes due to improper heat transfer.
Sometimes biological materials do not compress easily unless water is added.
The presence of water favours structure formation in the bonding of wheat straw
(Koullas and Koukios 1987) because the water acts as a binder in processing.
Faborode (1989) stated that an increase in moisture content of compacts produced
from barley straw resulted in an increase in the axial expansions. Rehkugar and
Buchele (1969) studied the compaction of wafering for moisture content in the range
of 6 to 25% (w.b.) and reported that the relaxed density of pellet decreased with
increasing moisture content. There were several studies reporting the effect of
moisture content on the durability of compacted biomass (Mohsenin and Zaske 1976;
Smith et al. 1977; Coates 2000). These studies reported that more durable compacts
were made in the moisture content range of 15 to 20% (w.b.) for alfalfa, wheat straw,
and cotton plant residues. Kubler (1987) reported that moisture content exceeding
20% dry basis (d.b.) may cause microbial growth in wood chips. Due to this, there is
11
degradation and self-heating during storage. According to Haussmann (1975),
briquettes made from sawdust, sander dust, shavings, peanut hulls, etc. with an
existing moisture content of 15% (w.b.) were dense and stable. Briquetting
switchgrass at a temperature of 25°C when moisture content increased from 10 to
15% (w.b.) resulted in relaxed densities that decreased from 40 to 30% (Kaliyan and
Morey 2006). Also, it was found that an increase in moisture content from 10 to 15%
(w.b.) resulted in increased durability of corn stover briquettes from 50 to 80% at an
applied pressure of 100 MPa.
In summary, it is clearly understood that the optimum moisture content for
densification is variable depending upon the type of raw material and pelleting
conditions.
2.1.4 Chemical composition
Biomass is composed of mainly cellulose, hemicelluloses, and lignin. The
chemical composition affects the ability to density of biomass material. Mani et al.
(2006b) determined the chemical composition of wheat straw, barley straw, corn
stover and switchgrass and found out the effects of the amount of protein and lignin
on the pelleting properties of all four biomass species. At high temperature all natural
binders such as protein, lignin, and starch become soft and may melt. Shaw and Tabil
(2006) reported the chemical composition of the different biomass samples including
peat moss, wheat straw, oat hulls, and flax shives. It was found that flax shives
contained protein (3.1%), ash (2.7%), fat (1%), crude fiber (56.9%), neutral detergent
fiber (82.3%), acid detergent fiber (67.6%), and lignin (17.0%). The percentage of
the components is expressed in a dry basis (d.b.). They found that peat moss had the
highest lignin content of 28.6% while flax shives had the lowest ash content of 2.7%.
12
2.1.5 Angle of internal friction and cohesion
It is important to have an accurate knowledge of the frictional behavior of
biomass grinds before designing the handling and storage equipment as well as
predicting flow behavior. The angle of internal friction and cohesion is one of the
mechanical properties of powdery materials and biomass grinds that need to be
considered when designing handling equipment. Seville et al. (1997) stated that the
coefficient of internal friction is related to the stress distribution within particles
under strain, while the coefficient of wall friction is related to the magnitude of the
stresses between the particle and the walls of its container. Another expression of the
angle of internal friction, the friction coefficient, of a variety of agricultural materials
has been reported by a number of researchers (Richter 1954; Snyder et al. 1967;
Thompson and Ross 1983; Chung and Verma 1989; Puchalski and Brusewitz 1996;
Molenda et al. 2000). According to Knowlton et al. (1994), flowability can be
described by cohesive strength, wall friction and compressibility.
There are a number of factors including moisture content, particle size and
shape, temperature, storage time, etc. that affect the wall friction of the biomass
grinds. Mani et al. (2004b) studied the effect of moisture content on the coefficient of
wall friction of corn stover grinds from a grinder with different screen sizes. They
found that the coefficient of wall friction increased significantly from 0.18 to 0.26 as
the moisture content increased from 7 to 15% (w.b.). This is partly explained by an
increase in adhesion between the grind and the steel surface with increased moisture
content. An increase in the coefficient of wall friction with an increase in the
moisture content of chopped hay and straw, grains and beans was similarly reported
(Wieneke 1956; Brubaker and Pos 1965; Snyder et al. 1967; Thompson and Ross
1983; Chung and Verma 1989).
13
2.2 Process Variables Influencing Biomass Densification
In addition to the raw material properties, pelleting process parameters such
as pressure, temperature, amount of binder used to form pellets and steam
conditioning have an effect on the mechanisms of compression and the quality of the
compacted materials. Also, die size and die speed have a significant influence on
pellet properties during and after densification.
2.2.1 Pressure
In commercial pellet mills, typical pressures applied to ground materials are
in the range of 50 to 150 MPa (Sitkei 1986). Due to high pressure, the biomass
components can change their properties and may act as binders and stabilizing agents
(Pickard et al.1961; Reece 1966; Rehkugler and Buchele 1969).
Mani et al. (2004c) studied the effect of compressive pressures between 31.08
and 136.77 MPa on the density of biomass pellets made from wheat and barley
straws, corn stover, and switchgrass using a single pelleter. It was found, for all
materials except the corn stover, that an increase in the compressive force resulted in
an increase in the compacted density of the pellets different screen sizes (0.8, 1.6,
and 3.2 mm). For corn stover grinds there was no significant effect on the pellet
density. They also reported that the maximum compact density was attained easily
for corn stover and less pressure was required for densification. Rhen et al. (2005)
investigated the effect of the pressure on the density and moisture absorption for
pellets produced from Norway spruce sawdust. It was concluded that for pressures
within the range of 46 to 114 MPa, there was very little effect on density and
14
moisture absorption. They also observed that the pressure in the die does not need to
exceed 50 MPa during the pelletizing process.
The density of straw briquettes has been found to increase almost linearly
with an increase in applied pressure (Reed et al. 1980; Singh and Singh 1982;
O’Dogherty and Wheeler 1984; Bhattacharya and Yeasmin 1984). Kaliyan and
Morey (2006) studied the effect of pressure on briquettes made from corn stover and
switchgrass using a piston cylinder apparatus. They concluded that increasing the
applied pressure within a range of 100 and 150 MPa at a temperature of 25°C and
moisture content of 10% (w.b.) increased the unit density and specific energy and
reduced the porosity of the briquettes. It was also reported that the durability of corn
stover briquettes, densified in the same condition, increased from 50 to 60%.
The optimum pressure for satisfactory pellets depends upon the physical and
chemical properties of the feedstocks.
2.2.2 Preheating temperature
Optimum preheating temperature of biomass is of importance for production
of acceptable physical quality of pellets in terms of hardness, moisture absorption,
tensile strength, and durability. It also affects the specific energy consumption in the
grinding and pelleting processes. Compacted products made at higher temperatures
result in lower force requirements (Hall and Hall 1968). Rhen et al. (2005) studied
the effect of temperature, ranging from 26 to 144°C, on pellets made from Norway
spruce sawdust. They reported that the dry density of pellets increased with higher
temperatures and lower moisture contents. In a temperature controlled extrusion
process using closed-ended die equipment, when the temperature was increased from
40 to 50°C at a moisture content of 14% (w.b), the power required to produce wafers
15
with the highest density was reduced (Orth and Lowe 1977). Smith et al. (1977)
investigated the effect of temperature on the stability and density of briquettes made
from wheat straw using a closed die unit. They found that when the temperature
increased from 80 to 110°C, both the stability and density of briquettes increased.
Reed et al. (1980) found that densified biomass (wood materials, agro-industrial
residues and agricultural residues) produced at temperatures around 220°C had
higher strength, density, and energy content than those produced at lower
temperatures. They also found that preheating the raw materials reduced the
compressive pressure. Kaliyan and Morey (2006) reported that increasing the
preheating temperature of corn stover and switchgrass from 75 to 100°C had the
potential to activate the natural binding components and resulted in briquettes that
were more dense and durable. They also found preheating temperatures in this range
resulted in less volume expansion for briquettes.
2.2.3 Binding of particles
Lignocellulose biomasses are mainly composed of lignin, cellulose, and
hemicelluloses that play an important role in the binding particles together in the
processing materials into pellets and briquettes. They also consist of natural binding
agents such as wax and protein that interact to help bond particles during
densification.
In lignocellulosic biomass, the amounts of lignin and glass transition
temperature of lignin will affect efficiency of densification. At the glass transition
temperature, the amorphous components of lignocellulosic material, lignin and
hemicelluloses, are activated, softened and act as a binder for the particles. Irvine
(1984) reported that the glass transition temperature of lignin was in the range of 60
16
to 90°C. Kaliyan and Morey (2006) found that the glass transition temperature of
lignin ranged from 62 to 101°C, while that of corn stover and switchgrass ranged
from 50 to 115°C. Several researchers (Gray 1968; Rumpf 1962; Tabil 1996;
Thomas and van der Poel 1996; Pietsch 2002; Shaw 2008) studied the binding of
particles of biomass feedstocks to produce densified products in the form of
briquettes and pellets. Knowledge of the binding mechanism and fundamental
compaction properties of biomass species is important for understanding the
densification process and to design the appropriate compaction equipment.
The binding mechanism between individual particles in densified products
consists of five different forces: solid bridges, attraction forces, mechanical
interlocking, adhesion and cohesion forces, and interfacial forces (Rumpf 1962;
Pietsch 2002). Solid bridges can be formed among molecular structures as well as
between particles because of high pressures and temperatures. Many attraction
forces, such as van der Waals’ forces, valence forces, and magnetic forces, are
created during compression and act to adhere particles. Flat-shaped and bulky
particles of raw material develop mechanical bonds by interlocking and creasing of
fibers during densification. Highly viscous binders such as molasses and tar adhere to
the surfaces of solid particles either by smoothing out surface roughness or by
allowing the intermolecular attractive forces to take in part in the binding
mechanism. Existence of moisture between particles can bond particles and cause
cohesive forces which are a result of interfacial forces and capillary pressure at the
liquid-gas interface.
According to Thomas and van der Poel (1996), the binding mechanism
consists of solid-solid interactions between particles, capillary forces among water,
17
air and solid material, adhesive and cohesive forces between particulates and binders,
and interactions between particles.
Mani et al. (2006b) reported that the binding mechanism for compaction
(pelletization) is comprised of particle rearrangement, elastic and plastic
deformation, and interlocking of particles. During particle rearrangement, a closely
packed mass is created without changing the original properties of the particles,
although, they are affected by wall friction. In the elastic and plastic deformation
stages, the particles are forced against each other as the applied pressure is increased.
As a result of this, there is an increase in the inter-particle contact area. Gray (1968)
stated that brittle particles add to mechanical interlocking and increase the overall
strength of the pellet. In these stages, bonding forces like van der Waal’s forces take
effect (Pietsch 1997).
Binders or stabilizing agents may be needed to bond particles when the
natural binding agents in the biomass, protein, lignin, and cellulose, are insufficient.
Therefore, with the addition of binders, a better quality of densified products can be
achieved, in terms of density, hardness, durability, and moisture absorption during
handling, transportation, and storage. Binders can be introduced to reduce the
springiness of raw material, to maintain maximum bulk density and to increase the
durability of the densified pellets (Sokhansanj et al. 2005). Tabil and Sokhansanj
(1996) and Pfost (1964) reported the most commonly used binders for pelleting of
animal feeds are calcium lignosulfonate, colloids, bentonite, starches, proteins, and
calcium hydroxide.
Increasing the amount of binders may result in an increase of relaxed density,
durability and shear strength of briquettes (Chin and Siddiqui 2000). The durability
18
of rice straw briquettes increased to 80% when they were produced with the addition
of molasses, sodium, and silicate in the range of 10 to 25% (by weight).
Clarke and Marsh (1989) added 5 to 20% (by weight) petroleum pitches to
increase the compressive strength of coal briquettes. The addition of 25% ground
almond hulls to rice straw produced cubes with 75% durability (Waelti and Dobie
1973).
2.2.4 Steam conditioning
In the pelleting process, steam conditioning is an important process parameter
which affects the density, hardness, and durability of pellets. Robinson (1984) stated
that steam conditioning contributes to the activation of natural and artificial binders,
gelatinization and natural lubricants of animal feeds during pellet manufacture.
There was also an increase in moisture and an improvement in feed conversion due
to steam conditioning.
During conditioning, dry or saturated steam can be added to ground materials
as a heat and moisture source. Steam conditioning may involve both vapor diffusion
and condensation. Vapor diffusion from the pressurized steam causes inter-particular
voids in the mash, while condensation of vapor on the surface of the mash changes
the thermal properties of the mash.
The effect of steam conditioning parameters on the quality of pellets has been
reported by a number of researchers (Dobie 1959; Smith 1959; Skoch et al. 1981;
Winowiski 1985; Hill and Pulkinen 1988; Tabil, 1996). Steam conditioning
temperature is an important parameter which affects the densification process and
quality of the final products.
19
Winowiski (1985) studied the conditioning temperature for a variety of feed
rations and suggested how to optimize the feed temperature during the conditioning
process prior to pelleting. According to Tabil (1996), the conditioning of alfalfa
grinds at temperatures of 92°C and above resulted in better quality pellets. It was also
found that increasing the conditioning temperature resulted in an increase in
durability and a decrease in energy consumption in the pelleting process. Using
steam conditioning, raising the temperature of alfalfa mash from 60 to 104°C
resulted in a 30 to 35% increase in alfalfa pellet durability (Hill and Pulkinen 1988).
Another important parameter that affects the densification of feedstock when
using the steam conditioning is moisture content. Hill and Pulkinen (1988) reported
that the moisture content of alfalfa from 3.5% to 8% (w.b.) did not affect pellet
durability. However, the power consumption of the pelleter was decreased by half
with an increase in moisture content due to a lubrication effect. In alfalfa grinds, the
moisture content of 8.5 to 10% (w.b.) was suitable for making pellets (Tabil 1996).
Steam conditioning time is also affects the physical quality of the densified
feedstocks, as well as, the energy consumption of the pellet mill. Tabil (1996)
reported that a steam conditioning time of 17 to 20 seconds was sufficient to achieve
the desired level of moisture and temperature for processing alfalfa pellets. In
pelleting animal feeds, increasing the residence time of the mash in the steam
conditioning chamber above 30 seconds slightly reduced pellet durability (Vest
1983).
2.2.5 Die geometry and die speed
In the pelleting process, die geometry and die speed significantly affect the
pellet densities, hardness, and durability and the specific energy required to form
20
pellets. Heffner and Prost (1973) studied the effect of die geometry on pellet
durability. They concluded that the die size with a higher length-to-diameter (l/d)
ratio resulted in an increase in durability of pellets. Tabil (1996) also studied the
effect of die geometry on alfalfa pellet durability. Length-to-diameter (l/d) ratios of
4.1 and 7.31 produced pellets with mean durabilities of 49.8 and 65.8%, respectively.
From the results, it was clearly seen that higher l/d ratios resulted in more durable
pellets. It was concluded that an increase in alfalfa pellet durability was a function of
the increased pressure and frictional heating of the ground particles in the die during
the pelleting process.
Die speed also influences pellet densification and the quality of pellets
produced using a pellet mill. Leaver (1985) stated that die speeds between 250 and
300 rpm are suitable for the production of small diameter pellets. Alfalfa pellets
produced at two die speeds (250 and 316 rpm) and a die with a hole diameter of 6.1
mm were considered by Tabil (1996). He observed that there is an increase in the
durability of the pellets created at the lower die speed. Pelleting using high die
speeds (501 and 565 rpm) was found to require high specific energy for the pellet
mill motor to complete densification. It was determined that the high rotational
speeds created more centrifugal force which affected the ability of the particles to
flow through the die.
2.3 Physical Quality of Densified Products
This section presents the important physical properties, including dimensional
stability, density, tensile strength (compression strength), hardness, durability, and
moisture absorption of densified products. Also, it discusses how these properties are
21
influenced by variation in the raw biomass material and in the parameters of the
processing equipment.
2.3.1 Dimensional stability and densities
In general, a change in the length and diameter of pellets is caused by
absorption of water and the breakage of bonds that were developed during the
compaction process. Many studies have been conducted with regard to the effect of
factors such as die temperature, particle size, and moisture content on the
dimensional stability of pellets produced from a variety of biomass feedstocks.
Shaw (2008) reported that the pellets formed from raw poplar and wheat
straw expanded in both the diametrical and longitudinal directions. In addition, pellet
expansion in both axes was decreased with an increase in the die temperature and a
decrease in the feedstock moisture content and particle size. Pellets produced from
peat moss, wheat straw, oat hulls, and flax shives had longitudinal expansions of
0.52, 2.59, 1.80, and 1.27% and diametrical expansions of 0.02, 0.61, 0.31 and
0.33%, respectively (Shaw and Tabil 2005).
Studies have shown that the preset load (compressive pressure), temperature,
moisture content and particle size affect the pellet density. Shaw (2008) reported that
the initial density of pellets formed at different conditions increased as the preset
load and temperature were increased and the particle size was decreased. It was also
found that the initial density of pellets formed from wheat straw increased as the
moisture content decreased from 15 to 9% (w.b.).
Using a laboratory-scale pellet mill with a die size of 4.76 mm, Colley et al.
(2006) found that the particle and bulk densities of pellets made from switchgrass
decreased from 16 to 24 % as the moisture content was increased from 6.3 to 17 %
22
(w.b.). There was a slight initial decrease in the porosity of the pellets reaching a
minimum of 51.61% at a moisture content of 8.62% (w.b.). A further increase in
moisture resulted in an increase in the porosity of the pellets.
2.3.2 Tensile strength
Due to static and dynamic forces during handling, transportation, and storage,
pellet attrition can be substantial. According to Thomas and van der Poel (1996),
attrition of pellets represents the gradual reduction of the pellet strength and consists
of fragmentation and abrasion. Particle size distribution of pellets has an impact on
both fragmentation and abrasion. Fragmentation involves cracks in pellets and fines
at the fracture area while abrasion involves cracks on the edge or surface-unevenness
of particles. Since the fines are produced due to abrasion, the bulk density of pellets
may increase. In other words, small particles fill the space in the large voids in the
middle of pellets, and therefore, the bulk density increases.
Tensile strength is one indicator of pellet quality and can be determined by a
diametrical compression test. Tensile strength is affected by the processing
temperature of equipment, moisture content and particle size of the feedstock. Higher
tensile strength indicates higher quality pellets.
According to Shaw (2008) increasing both processing temperature (70-
100°C) and moisture content (9-15% w.b.) resulted in an increase in the tensile
strength of untreated poplar and wheat straw pellets. When formed using hammer-
mill screen sizes of 0.8, 1.6 and 3.2 mm, the tensile strength of the pellets increased
from 0.45 to 1.28 MPa for the poplar and from 0.47 to 1.33 MPa for the wheat straw.
Decreasing the particle size (i.e. screen) showed a significant increase in the tensile
strength of the untreated feedstock pellets.
23
An increase from 0.44 to 2.47 MPa in the tensile strength of alfalfa pellets
was reported by Tabil and Sokhansanj (1997) when preset compressive loads of 500-
4400 N were used. The alfalfa grinds were classified into three groups, namely high,
medium, and low quality grinds in accordance with leaf-to-stem ratio and weed
content. High quality alfalfa grinds had high leaf-to-stem ratios and low weed
content, while low quality alfalfa grinds had low leaf-to-stem ratios and high weed
content. Pellets produced from the high quality alfalfa grinds had the highest tensile
strength (2.47 MPa) using preset loads of 3000 N, whereas pellets formed from
medium quality grinds had the lowest tensile strength (0.44 MPa) using preset loads
of 500 N.
2.3.3 Hardness
Hardness represents the amount of force that is needed to fragment pellets. A
number of test devices for hardness have been developed, such as the Kahl device,
Schleuniger test apparatus, Pendulum pellet hardness test device and the Universal
compression test device. These were reviewed by Thomas and van der Poel (1996).
An increase in moisture content from 6.3 to 17% (w.b.) resulted in a decrease
in hardness from 30.21 to 21.6 N for switchgrass pellets (Colley et al. 2006). The
reason for this is probably due to moisture disrupting particular bonds formed during
the compaction process. Alfalfa pellets produced without binders from intermediate
and high quality chops had the higher hardness compared to pellets from low quality
alfalfa (Tabil 1996).
24
2.3.4 Durability
Durability is the most important property for evaluating the quality of
biomass compacts (pellets, briquettes, cubes). The durability of pellets describes their
ability to resist the effects of impact force and vibration generated during handling
and transportation (Mani et al. 2006a). Thus, it is a good indicator how well the
material resists forces that cause the material to be dusty and crumbly during
handling and storage.
Several devices are available for the evaluation of the durability of pellets.
Studies of Thomas and van der Poel (1996) discussed different instruments including
the Pfost tumbling can, Holmen tester, and sieving device. The tumbling can method
can be used with variations in the speed of tumbling, length of tumbling time, sieve
size and the amount of samples tumbled (Richards 1990; Raghavan and Conkle
1991). For determining the durability of very hard pellets such as dairy feed pellets,
the tumbling can method can be modified by adding steel nuts, bolts, or ball bearings
along with the pellets before tumbling (Winowiski 1998).
McKee (1990) studied the durability range and testing time for both the Pfost
tumbling can and Holmen tester. The results of his research showed that for the Pfost
tumbling can, there was a curvilinear decrease in durability ranging from nearly 98 to
91% with testing times up to 20 min, while for the Holmen tester, durability was in a
range from 95 to 60% with times up to 5 min. Therefore, it was clearly shown that
the Holmen tester gives durability in a wider ranger with a shorter testing time
compared to the Pfost tumbling can.
Tabil (1996) studied the durability of alfalfa pellets at three different qualities
of chops using both Dural and Stein breakage testers and found pellets made from
high quality chops were the most durable. Rhen et al. (2005) used compressive
25
strength as a measure of durability. They found that the durability of Norway spruce
pellets increased with an increase in temperature and a decrease in moisture content.
Durability of switchgrass pellets was studied in a moisture range of 6.3 to 17
% (w.b.) (Colley et al. 2006). Initially, there was an increase in durability as the
moisture increased, since the binding force of the water molecules strengthened the
bond between individual particles in the pellets. Maximum durability (96.65%) was
found to occur at a moisture content of 8.62% (w.b.). Beyond a moisture content of
8.62% (w.b.) particle bonds were disrupted and the durability of the pellets decreased
to 78.44%. The maximum durability of straw wafer was obtained at moisture
contents ranging from 13 to 17% (w.b.) (O’Dogherty and Wheeler 1984).
2.3.5 Moisture absorption
Generally, an increase in air temperature and relative humidity will cause an
increase in the moisture absorption of pellets during storage and handling. It was
stated by Kalliyan and Morey (2006) that transportation and storage in rainy or high
humidity conditions may affect the quality of densified products.
Colley et al. (2006) reported that moisture absorption of the pellets decreased
with a decrease in relative humidity. It was found that the minimum moisture
absorption occurred at a temperature of about 90°C (Rhen et al., 2005). For Norway
spruce pellets, they found that low initial moisture content resulted in maximum
moisture absorption and concluded that a period of 25 h is enough for the
conditioning of pellets.
Tabil (1996) studied the effect of absorbed moisture on pellets made from
different qualities of alfalfa chops. An environmental chamber with a relative
26
humidity of 90% and temperature of 28°C was used. In this study, pellet quality was
described in terms of durability, hardness, and volume expansion. Pellets produced
from intermediate quality chops took longer to absorb moisture among the pellets
from other quality chops. There was a significant decrease in pellet durability when
pellet moisture content was above 10% (w.b.). It was also observed that pellet
hardness was sensitive to moisture changes and decreased significantly as the
moisture increased above 8% (w.b). For the volume expansion of pellets made from
low and intermediate chops, moisture contents of 8 or 10% (w.b.) and above resulted
in a significant increase in the volume.
2.4 Specific Energy Requirement and Economics
In the densification process, the total specific energy consists of both the
energy required to grind the raw material and the energy needed to form pellets from
the ground materials. Therefore, this section presents research regarding the specific
energy of biomass grinding using a hammer mill and the energy required to form
pellets from biomass using a single-pelleter. In addition, the economics of pelleting
flax shives is reviewed.
2.4.1 Specific energy required to grind biomass
The specific energy required to grind biomass is affected by the particle size,
and moisture content of the raw materials, as well as type of grinder used. Schell and
Hardwood (1994) found that grinding lignocellulosic biomass using a hammer mill
requires less specific energy than disc milling. Mani et al. (2004a) evaluated the
specific energy required for the grinding of four biomass, namely wheat straw, corn
27
stover, switchgrass and barley straw using a hammer mill with three different screen
sizes (0.8, 1.6, and 3.2 mm). The specific energy required to grind all feedstocks was
significantly increased with a decrease in the particle size. According to Datta
(1981), coarse grinding (0.2-0.6 mm) of hardwood chips required 72 to 144 MJ/t,
whereas fine grinding (0.15-0.3 mm) of hardwood chips demanded 360 to 720 MJ/t
of specific energy. However, it was not reported what type of mill was used. Shaw
(2008) determined the specific energy required to grind feedstocks, raw poplar and
wheat straw, using a hammer mill with screens of 3.2 and 0.8 mm. The specific
energy required to grind wheat straw was in a range from 41 to 284 MJ/t. From this
result, it is clearly seen that more specific energy is required when the particle size of
wheat straw is reduced. The initial moisture content of the grinds was 11.7 % (w.b.).
The specific energy required for grinding of all four biomass increased with
an increase in moisture content (Mani et al. 2004). Grinding of switchgrass with the
use of a hammer mill with a screen size of 3.2 mm at a moisture content from 8 to
12% (w.b.) required the highest specific energy from 86 to 99 MJ/t, whereas grinding
of corn stover using the same screen size at a moisture content from 6.2 to 12% (w.b)
consumed the lowest specific energy from 25 to 40 MJ/t. This may be explained by
an increase in the shear strength of the biomass as the moisture content of the
feedstock is increased.
However, Jannasch et al. (2001) reported that grinding switchgrass at
moisture contents of 10 to 12% (w.b.) using a commercial hammer mill with screen
sizes of 5.6 and 2.8 mm both required the same energy of 201 MJ/t when throughput
was about 2 t/h.
28
2.4.2 Specific energy required to form pellets
In general, total specific energy depends upon a variety of parameters
including the pre-set load (i.e., compression pressure), processing temperature,
moisture content, and particle size of the grinds. The total specific energy consists of
the specific energy required for compression and the specific energy required to
extrude the grinds during densification using single pelleting equipment.
Shaw (2008) found that only a small amount of the total specific energy is
required to extrude the grinds, while a large percentage of the total specific energy is
used for compression of the feedstocks. It was found that the specific compression
energy was 7.2 and 39.1MJ/t at pre-set loads of 1000 N (31.6 MPa) and 4000 N
(126.3 MPa) for pretreated wheat straw and untreated wheat straw, respectively. This
supports the assertion that an increase in the pre-set load results in an increase in the
specific energy required for compression.
In general, increasing the preheating temperature of biomass results a
decrease in the specific energy consumption. According to Sokhansanj et al. (2005),
increasing the process temperature caused a significant decrease in the specific
energy required to compress the grinds. In this study, grinds at processing
temperatures of 70 and 100°C and moisture content levels of 9 and 15% (w.b.) were
used to form pellets. There was not a significant effect on the specific compression
energy of the feedstocks. It was also found that an increase in the moisture content of
the feedstocks resulted in a decrease in the specific energy required for compression.
For extrusion, the mean value of specific energy required to extrude wheat straw
pellets was in a range from 0.25 to 0.78 MJ/t.
Mani et al. (2006a) reported that the total specific energy for pelleting corn
stover ranged from 12 to 30 MJ/t including both the compression and extrusion
29
energies. Compacting corn stover into the form of briquettes was performed at
moisture contents of 5, 10, and 15% (w.b.) and compression pressures of 5, 10, and
15 MPa. Specific compression energies of 8.1 to 7.3 MJ/t were reported using a pre-
set pressure of 5 MPa at the high moisture content levels of 10 and 15% (w.b.),
respectively.
2.4.3 Economics of pelleting
A number of issues are considered regard to the economic value of pellets
produced from flax shives. The total amount of flax produced yearly in
Saskatchewan or in Western Canada is of importance. Ulrich (2008) stated that there
are about 639,000 tonnes of salvageable oilseed flax straw produced annually in
Canada. Schweitzer-Mauduit, Canada purchases from 80,000 to 120,000 tonnes of
straw annually for the processing tow fiber used in cigarette papers. Shive content is
estimated to be about 78% of flax straw and the average price of shives is assumed to
be $2/tonne, based on a 10 year average of oilseed flax straw. These statistics show
that there would be abundant source of heat produced if waste flax shives were
converted into biofuel. The cost of this solid fuel is relatively low when compared
with other sources of energy such as crude oil, coal, and natural gas.
2.5 Emissions
The environmental issues related to biomass fuels must be considered.
Biofuels are more carbon-friendly than fossil fuels as the carbon released has been
recently removed from the air. The burnig of fossil fuels may cause more adverse
30
health effects than the use of biofuels. When biofuels are burned, inorganic ash and
particulate matter remain, and gases, such as carbon monoxide, are released. When
compared to coal, biomass fuels have lower levels of carbon, mineral ash, higher
alkali levels and higher levels of volatile products (Demirbas 2004). The important
chemical properties, such as the proximate and ultimate chemical analysis and the
heating value of the volatiles, were reviewed for different biomass fuels (Demirbas
2004). Several studies reported that softwood pellets are an environmentally friendly
fuel with low emissions (i.e., no serious effect on the environment). Therefore; they
could be used in places where the population is large (Boman et al. 2003; Olsson et
al. 2003; Johansson et al. 2004; Olsson and Kjallstrand 2004). Olsson (2006) studied
the emissions for fuel pellets made of wheat straw and peat and compared the
amount released to those from the combustion of softwood pellets. It was found that
pellets made from wheat straw and peat/wood had low emissions during combustion.
Boman et al. (1998) performed a study of the products of combustion of six biofuels
that were derived from products of the logging industry. From the results of this
study, it would appear that biofuels would be less harmful to health than coal or
petroleum products.
2.6 Summary
Biomass materials including straw saw dust, shavings, corn stover,
switchgrass, and many others have been studied extensively as a source of biofuel.
There is a need for densification of these materials to reduce the difficulties of
handling and the costs for storage and transportation. However, no research work on
the densification of flax shives is found in the literature. Densification of various
31
types of biomass is affected by both raw material properties and process variables.
Important raw material properties such as particle size, particle size distribution, bulk
and particle density, moisture content constituents, and chemical components were
studied on various biomass materials. The quality of densified products made from a
variety of biomass feedstocks was also studied extensively, considering parameters
such as bulk and particle density, hardness, durability, and moisture absorption.
Particle size reduction of biomass materials is needed before processing them
into pellets by densification. Particle size and particle size distribution affect the
material properties and binding characteristics of particles during densification.
Decreasing the particle size and moisture content resulted in higher bulk and particle
density and more particle surface area availabe for binding particles. A wide particle
size distribution is more suitable for the compaction process. At high temperatures,
natural binders, including proteins and lignins, become soft and help to bond the
particles together.
Process variables including pressure (load), preheating temperature, bonding
agents, steam conditioning, die geometry, and die speed affect biomass densification.
At high pressure, the biomass components change their properties and act as a
binding agent for particles. An increase in an applied pressure resulted in an increase
in density of corn stover pellets. Increasing the applied pressure above 150 MPa
resulted in an increase in unit density and durability of corn stover briquettes. The
preheating temperature, when increased from 80 to 110º C, resulted in an increase in
both stability and density of wheat straw briquettes.
Natural binding components were activated when the preheating temperature
increased from 75 to 100º C for corn stover and switchgrass and resulted in higher
density and durability and less volume expansion for the briquettes. If natural
32
binding agents are not sufficiently present, additional binders are needed to bond
particles together when processing them into pellets and briquettes. Binders
including calcium lignosulfonate, colloids, bentonite, starches, proteins, and calcium
hydroxide are used to maintain maximum bulk density and to increase the durability
of pellets for animal feeds.
Steam conditioning contributes to the activation of natural and artificial
binders by causing gelatinization at an increased conditioning temperature. Alfalfa
grinds at a temperature of 92º C and above resulted in better quality pellets. In
addition, steam conditioning resulted in increasing the moisture content by about 2%.
Reduction of particle size of the biomass increased the specific energy
required for grinding. However, increasing the preheating temperature and moisture
content of the feedstocks to form pellets resulted in a decrease in the specific energy
required for compression. Biomass fuels had lower levels of mineral ash, higher
levels of alkali and volatile products when compared to coal. Softwood, wheat straw,
and peat pellets are environmentally more friendly fuels due to their low emissions
during combustion.
33
3. MATERIALS AND EXPERIMENTAL METHODS
The coarse flax shives and ground shives used in this research were obtained
from two flax straw processing companies. This chapter presents the physical and
chemical properties of the shives and of the waste canola meal used as a binding
agent. In addition, the methodology and equipment used to determine the raw
material properties, the methods followed for determination of the properties of the
solid fuel pellets and, the experimental designs used for characterization of the flax
shive material and production of the fuel pellets are described.
3.1 Materials and Characterization
Flax shive material was obtained from two flax straw processing companies,
Biofibre Industries Inc. and Biolin Research Inc. First, the physical and chemical
properties and the heat of combustion of the coarse raw material and prepared ground
material were determined. Then, the ground material was processed into biofuel
pellets using the single pelleter and a pilot-scale pellet mill. For the fuel pellets that
were produced using the pilot-scale pellet mill, canola meal used as a binder obtained
from the University of Saskatchewan, was added to improve the quality of the
pellets. The physical properties and chemical composition of the canola meal were
also determined in the course of this research. The coarse flax shives and canola meal
are shown in Figure 3.1.
34
(a) (b)
Figure 3. 1 Materials: (a) coarse flax shives, (b) canola meal used as a binder.
A representation of the overall material characterization for the ground flax
shives is given in Figure 3.2.
Figure 3. 2 Material characterization of flax shives.
The flax shives from Biofibre Industries Inc. were characterized in terms of
particle size and distribution, moisture content, particle density, and bulk density.
Flax shives
Bulk density
Moisture content
Particle size analysis
Heating value Physical properties Chemical composition
Particle density
Porosity
Ash
Lignin
Fat (Ether Extract)
Crude fiber
Angle of friction and cohesion
Hemicelluloses
Cellulose
Specific heat capacity
Protein
35
The goal of the experiments on these was to evaluate the effect of grinding (i.e.,
particle size of the flax shives) on the density and durability of the pellets. The
pellets were produced using single-pellet equipment. The raw material contained
some large pieces of straw that had not been broken into fiber and shives. Therefore,
it was necessary to remove these large particles before processing. For the
preparation of the material, the flax shives were cleaned to separate fine linen fiber
from coarse material before grinding. To perform this operation, a forage separator
analyzer was used. This piece of equipment has a screen with 3.8 mm square
openings. The particles which passed through the screen were then ground using a
Buhler hammer mill (Buhler Manufacturing, Winnipeg, Morden, MB) with screen
sizes of 2.4, 3.2 and 4.8 mm. Finally, for this set of experiments, the flax shive grinds
were characterized and then densified using a single pelleter.
The flax shives from Biolin Research Inc. were characterized in terms of
physical and chemical properties, as well as, heating value. The flax shives were
already quite clean in a raw form. Therefore, cleaning and separating the fiber from
the raw form was unnecessary. The goal of the experiments on these shives was to
determine the effects of moisture content, particle size and the addition of canola
meal on the pellet quality. The flax shives were ground in the same hammer mill as
above, however, with screen sizes of 3.2, 4.8 and 6.4 mm. They were then pelletized
using a laboratory-scale pellet mill. To determine the specific heat capacity, the flax
shives were ground using a Thomas Wiley Laboratory Mill (Model 4, Thomas
Scientific, Swedesboro, NJ) with a screen size of 1.0 mm.
The canola meal, which was only added to the shives from Biolin Research
Inc., was ground using the Buhler hammer mill with a screen size of 3.2 mm. Some
36
physical and chemical properties of the canola meal were determined prior to mixing
with the flax shive grinds.
3.1.1 Particle size and size distribution
Initial particle length of flax shives in the raw form from both sources was
determined by ASABE Standard, ANSI/ASAE S424.1 MAR1992 (R2007). The flax
shives from both sources were ground to give a size distribution more suitable for
manufacturing the pellets as a biofuel. To determine the geometric average particle
size of the grinds, a Ro-Tap sieve shaker (W.S. Tyler Inc., Mentor, OH) was used.
The particle size distribution of the flax shives was determined according to ASAE
Standard S319.3 (ASAE, 2005) and the experiment was replicated three times, once
for each of the three hammer mill screen sizes. For the grinds from Biofibre
Industries Inc., a stack of sieves arranged from the largest to the smallest openings
was placed in the shaker. The sieves had Canadian series sieve numbers of 16, 20,
30, 40, 50, 70 and 100 with nominal openings of 1.188, 0.841, 0.595, 0.420, 0.297,
0.210 and 0.149 mm, respectively. For the shives from Biolin Research Inc., the
particle size and distribution of the grinds was determined using the Canadian series
sieve numbers of 4, 6, 8, 12, 16, 20, 30, 50, 70, 100, 140, 200 and 240 with nominal
openings of 4.760, 3.366, 2.376, 1.612, 1.188, 0.841, 0.595, 0.297, 0.210, 0.149,
0.105, 0.074 and 0.052 mm, respectively. Approximately 100g of ground material
were used and the shaker was run for 10 minutes. After sieving, the particles
obtained on each sieve were weighed. The geometric mean particle size (dgw) and
geometric standard deviation (Sgw) for the log-normal distribution on a weight basis
were calculated as follows:
37
∑
∑=
=
=−
n
1ii
n
1iii
1
gw
W
)dlog(Wlogd (3.1)
2
1
n
1ii
n
1igwii
log
W
)logdd(logWS
∑
∑ −=
=
=
(3.2)
])Slog(Slog[d2
1S 1
log
1
log
1
gwgw
−−− −≈ (3.3)
where di = nominal sieve aperture size of the ith sieve (mm),
di+1 = nominal sieve aperture size in next larger than ith sieve (mm),
dgw = geometric mean particle size or median size of particles by mass (mm),
Slog = geometric standard deviation of log-normal distribution by mass in ten-
based logarithm, dimensionless,
Sgw = geometric standard deviation of particle size by mass (mm),
Wi = mass on ith sieve (g), and
n = number of sieves +1 (pan).
3.1.2 Bulk and particle density
The bulk densities of the coarse and ground shives from both sources and the
ground canola meal were determined using the grain bulk density apparatus. A
standard 0.5 L (500g) steel cup (SWA951, Superior Scale Co. Ltd., Winnipeg, MB)
was filled using a funnel. To maintain a continuous flow, a thin steel rod was used.
The cup was then gently levelled, with a rubber-coated steel rod, and weighed. The
measurements were repeated three times for all samples. The bulk density was
calculated as
Vc
mb =ρ (3.4)
38
where bρ = bulk density (kg/m3),
m = mass of sample in the cylinder (kg), and
Vc = volume of cylinder (m3).
To measure the particle density of the ground flax shives and ground canola
meal, a gas pycnometer (Quantachrome Corp., Boynton Beach, Fla.) was used. The
particle densities of the three grinds produced from both sources were determined.
Prior to the particle density measurements, the pycnometer was calibrated using a
large spherical ball of known volume. To determine the particle densities of the
samples, first, a reference volume of nitrogen gas was pressurized to about 117.2 kPa
(P1). The gas was then allowed to flow into the sample cell until a constant pressure
P2 was reached. By measuring the pressures, P1 and P2, the volume of solid Vs may
be calculated by
−−=
12
1Rcells
P
PVVV (3.5)
where Vs = volume of solid (cm3),
Vcell = volume of the cell (cm3),
VRR = reference volume for the large cell (cm3),
P1= pressure reading after pressurizing the reference volume (kPa) and
P2= pressure reading after including volume of the cell (kPa).
After the volumes for all of the samples were determined, their masses were then
determined. The particle densities of the ground flax shives and canola meal were
calculated using
s
tV
m=ρ (3.6)
where ρt = particle density (kg/m3),
m = mass of sample in the cylinder (kg), and
Vs = volume of solid (m3).
39
An average particle density was obtained from three trials. Porosity of the shive
samples was calculated using
1001 ×
−=
t
b
ρ
ρε (3.7)
where ε = porosity (%),
ρb = bulk density (kg/m3) and
ρt = particle density (kg/m3).
3.1.3 Moisture content and conditioning
The moisture contents of the coarse and ground flax shives, the canola meal,
the final mixture of shives and canola meal, and the pellets produced from the shives
were determined according to ASAE Standard S358.2 (ASAE, 2005), as this is the
standard method for forages. This method involves determining the reduction in
weight of samples, after oven drying for 24 h at 103°C. A forced convective electric
oven (Blue M Thermal Products Solutions, Williamsport, PA) was used and three
replicates were performed for each sample. The flax shive samples were first
conditioned to various moisture levels as required for different measurements. For
example, to measure the specific heat capacity of the shives, samples were
conditioned to 8, 11, and 14% (w.b.). The mixtures of shives and canola meal prior
to producing pellets were conditioned to 8, 11, and 14% (w.b.). The following
equation was used to determine the amount of water that was required to achieve the
desired moisture content of samples.
wf
wiwf
iwM
MMmm
−
−=
1 (3.8)
where mw = mass of water added to sample (g),
mi = initial mass of sample (g),
40
Mwf = final desired moisture content of sample (w.b.), and
Mwi = initial moisture content of sample (w.b.).
After adjusting the moisture content, the samples were stored in sealed bags and
allowed to equilibrate for 24 hours at room temperature before being subjected to
measurement and processing.
3.1.4 Chemical components
The chemical compositions of the flax shives from Biolin Research Inc. and
the canola meal were determined in this research. Protein, ash, fat and crude fiber
contents of the samples were determined at Sun West Food Laboratory, Saskatoon.
Determination of ash content, crude fiber content and protein content was performed
according to AOAC methods of 942.05 (AOAC, 2005), 992.23 (AOAC, 2005) and
962.09 (AOAC, 2005), respectively. The standard method of AOCS Am 2-93 (2000)
was used to determine the fat content of samples. The amounts of acid detergent fiber
(ADF), neutral detergent fiber (NDF) and acid detergent lignin (ADL) were
determined in the Plant Sciences Department at the University of Saskatchewan
using the ANKOM 200/220 Fiber Analyser (ANKOM Technology, Fairport, NY). In
addition, cellulose and hemicelluloses of the samples were calculated by
ADLADFCellulose −= (3.9)
ADFNDFosesHemicellul −= (3.10)
The values chemical components are expressed by weight in percentage dry basis
(d.b.).
41
3.1.5 Angle of internal friction and cohesion
Since there is no information on the frictional properties of grinds from flax
shives, the frictional characteristics of shives on a steel surface were determined in
this study. Friction and cohesion are important as they have an effect on the power
needed to handle the raw material and this affects the design of equipment. A
Wykeham Farrance shear box apparatus (Wykeham Farrance International Ltd.,
Slough, UK) shown in Figure 3.3 equipped with a 100 mm square shear box and
motor assembly was used to determine the angle of internal friction and cohesion of
the flax shives obtained from Biolin Research Inc. Both the top and bottom boxes of
the apparatus were filled with a sample. The bottom box was then pulled horizontally
at a constant speed of 0.4 mm/min. Shear stress at four different normal loads of 100,
200, 300, and 400 N was applied to the shives via a load hunger and was measured in
three replicates. The method and calculation of the coefficient of internal friction and
cohesion of the flax shive sample were conducted, as explained by Peleg (1977) and
Tabil and Sokhansanj (1997).
στ ic φ tanC += (3.11)
where τ = shear stress (kPa),
Cc= cohesion (kPa),
φi= angle of internal friction; and
σ= normal stress (kPa).
42
Figure 3. 3 Wykeham Farrance shear box apparatus.
3.1.6 Specific heat capacity
Specific heat capacity is a function of temperature and moisture content, and
is an important parameter for characterizing the flax shives obtained from Biolin
Research Inc. For the measurement of specific heat capacity of flax shives,
Differential Scanning Calorimetry, DSC 111 (Setaram Scientific & Industrial
Equipment, Caluire, France) was used and is shown in Figure 3.4.
Figure 3. 4 Differential Scanning Calorimetry, DCS 111 for specific heat capacity measurement of samples.
43
Yang et al. (2002) and Izadifar and Baik (2007) reported that DSC is an
accurate and rapid method for measuring specific heat capacity.
Flax shives were ground using a laboratory grinder (Laboratory Mill Model
4, Thomas Scientific Co., USA) with a screen size of 1 mm. The specific heat
capacities of flax shives with three different levels of moisture content, i.e., 8, 11,
and 14% (w.b), were determined. About 10 mg of flax shives were placed in the
DSC machine and sealed in an aluminum pan along with a blank reference pan.
During the measurement, the temperature was increased from 25 to 80°C at a heating
rate of 5°C/min and three replicates were performed. The specific heat capacity was
calculated using
=
Mdt
dTdt
dH
Cp (3.12)
where dH/dt = heat flow rate (J/s),
M = sample mass (kg),
Cp = specific heat capacity (J/(kg°C)),
t = time (s) and
T = sample temperature (°C).
3.1.7 Combustion energy
The combustion energy of the flax shives obtained from Biolin Research Inc.
was measured by a bomb calorimeter (Series 1300 Plain Calorimeter, Parr
Instrument Company, Moline, IL) which is shown in Figure 3.5. The bomb
calorimeter consists of an oxygen combustion bomb, water container, ignition switch
and gas cylinder (Figure 3.6). The thick steel bomb contains a sample pan and
ignition wire.
44
Calibration of the bomb calorimeter used benzoic acid. First, the water
container was filled with about 2 liters of distilled water. Then, a 1g pellet of benzoic
acid, with a known caloric value of 26.5 MJ/kg, was placed into the stainless steel
sample pan. A 10 cm long fuse wire touched the pellet and ignited the benzoic acid
in the bomb calorimeter. A thermometer was connected to a Campbell data logger
(Model CR10X, Campbell Science, Inc. Logon, UT) to record the water temperature.
Figure 3. 5 Bomb calorimeter set up.
45
(a) (b)
Figure 3. 6 Bomb calorimeter; (a) Gas cylinder and oxygen bomb, (b) Bomb calorimeter in water container with ignition switch, and thermometer.
Prior to measurement, the water was allowed to reach an equilibrium condition.
After completion of the measurements, the unburned fuse wire was removed and the
length was measured. The change in the temperature of water indicates the energy
that was released by combustion, and 2.3 cal/cm of wire burned needs to be
subtracted from the calorie reading as this is the amount of energy.
The energy content of the shives in pellet form was evaluated according to
ASTM Standard E 711 (ASTM, 2003), the accepted test method for the combustion
energy of coal and coke. The energy content for the samples was calculated using
( )am
eWtH
−= (3.13)
( )m
eWtHg
−= (3.14)
where W = energy equivalent of calorimeter (J/°C),
Hg = energy content of standard benzoic acid (J/kg),
ma = mass of benzoic acid (kg),
46
t = corrected temperature rise (°C),
e = correction for heat of firing fuse wire (J),
m = mass of sample in combustion cap (kg) and
H = energy content of sample (J/kg).
3.2 Material Processing Equipment and Procedures
Once the experiments on the flax shives were complete, the pelleting process
was performed using a closed-end die and plunger unit for the shives from Biofibre
Industries Inc. and a pilot-scale pellet mill for the shives from Biolin Research Inc.
3.2.1 Pelleting with the single pelleter
The single pelleting equipment is shown in Figure 3.7. The single pelleter
consists of a die and plunger assembly and stainless steel base (supporter) along with
a heating element.
Figure 3. 7 Single pellet equipment.
47
The plunger and die both had diameters of 6.35 mm. The die was 135.24 mm
long and had a heating element wound around it. To control the temperature, two
type-T thermocouples were installed in the die wall and attached to a temperature
controller. A stainless steel base was used to support the entire assembly and provide
a rigid surface upon which to compress the sample.
The single-pelleter was constructed for use in the Instron® model 1011
testing machine (Instron Corp., Canton, MA) as shown in Figure 3.8. The Instron
testing machine was used to control pressure, and the cross-head speed of the plunger
during pelleting. The preheating temperature of the biomass was controlled by the
heating element. To produce the pellets, constant pressure of 139 MPa, temperature
of 100˚C and moisture content of 10% (w.b.) were used. The preset load was 4400N.
Approximately 0.5g of shives was placed in the single pelleter die chamber prior to
loading and the die was heated to 100˚C. The ground flax shives were then
compressed by the plunger with a crosshead speed of 50 mm/min. When the preset
load was reached, the crosshead on the plunger was stopped.
Figure 3. 8 Instron® model 1011 testing machine with attached single pelleter.
48
The pellet was removed from the chamber by application of a gentle force. Grinds
from three different screen sizes (i.e., 2.4, 3.2 and 4.8 mm) were formed into pellets
using the single-pelleter and nine pellets from each particle size were produced.
These pellets were produced to study the effect that the particle size has on the unit
density and durability of individual pellets.
3.3.2 Pelleting with the pilot-scale pellet mill
After grinding and combining with set amounts of canola meal, the flax
shives were processed into pellets, using a California Pellet Mill (CPM-Laboratory
Model CL-5, California Pellet Mill Co., Crawfordsville, IN) without steam
conditioning, as shown in Figure 3.9. A chart of the complete process for producing
fuel pellets from the raw flax shives, as supplied, is given in Figure 3.10. Prior to the
pelleting process, the required amount of canola meal was calculated and mixed with
ground shives using a cement mixer. After mixing, the moisture content was
determined and moisture conditioning was performed using distilled water.
Figure 3. 9 California Pellet Mill.
49
Figure 3. 10 Processing scheme of flax shives for fuel pellets.
In this experiment, the pilot-scale CPM had a motor power 1.5 kW (2.0-hp)
and a die and roller assembly consisting of two main parts: a die, with 6.4 mm holes
and a length of 46.8 mm, and a roller. These are shown in Figure 3.11.
(a) (b)
Figure 3. 11 Die and roller assembly; (a) Die, (b) Roller.
Mixing materials
Moisture adjustment
Moisture content determination
Pelleting process
Screening
Size reduction
Canola meal Raw material (Shives)
50
To produce the pellets, about 0.9 kg of ground flax shive sample was placed
in the vibratory feeder and the flow rate was controlled by the feeder chute located
on the control panel. In the pelleter, the die and roller rotated in opposite directions
which created frictional heating by high pressure and force. The ground raw
materials (i.e., the mixture of flax shive grinds and canola meal) were densified
through the open-ended cylindrical die from the inside of the ring towards the outside
of the ring and compacted particles were formed which were compressed against
each other. The feedstock was then cut off with a knife at lengths of less than 30 mm.
After pelletizing, the pellets were cooled to room temperature.
The purpose of the experiments performed on these pellets was to investigate
the effects of the moisture, particle size of flax shives and the addition of three levels
of canola meal on the properties of pellets produced using the pilot scale pellet mill.
Therefore, the quality of the pellets, in terms of bulk and particle densities,
dimensional stability, hardness, durability, and moisture absorption were studied
using a Central Composite Face-Centered Design (CCFD). This type of design helps
to optimize the estimation of the effects of the independent variables with a
minimum number of experimental points and consists of a 3k factorial, where k is the
number of levels in each of 3 factors. Some of the 3k combinations of runs were left
out. Analysis of variance and regressions were used to find estimates of the effects
of independent variables on the dependent variables and to test hypotheses about the
significance of these effects. The assumptions for least squares methods are that 1.)
the treatment and environmental effects are additive, and that 2.) the experimental
errors are randomly distributed, independent and normally distributed and with a
common variance.
51
For the pellets produced using the CPM, the three factors that were varied
were screen size for the grinds, moisture content and amount of canola meal added.
To grind the flax shives, the hammer mill used screen sizes of 3.2, 4.8 and 6.4 mm.
The samples had moisture contents of 8, 11 and 14% (w.b.) and canola meal
amounts of 18, 21 and 24% by weight of the total mixture. The CCFD specified the
20 experimental runs as given in Table 3.1.
Table 3.1 Central Composite Face-Centered Design (CCFD).
Run Screen size
(mm)
Moisture content
(% w.b.)
Canola meal (%)
1 3.2 8 18
2 3.2 14 18
14 3.2 11 21
3 3.2 8 24
4 3.2 14 24
12 4.8 11 18
10 4.8 8 21
9 4.8 11 21
11 4.8 14 21
13 4.8 11 24
16 4.8 11 21
17 4.8 11 21
18 4.8 11 21
19 4.8 11 21
20 4.8 11 21
5 6.4 8 18
6 6.4 14 18
15 6.4 11 21
7 6.4 8 24
8 6.4 14 24
52
3.3 Quality Properties of Pellets
Several experimental tests were used to evaluate the physical and fuel
properties for the fuel pellets made from flax shives. These tests are listed in Table
3.2 along with the equipment that was used to perform the tests. For emission
measurements of the flax shive pellets, emissions from commercial wood pellets
were also measured for comparison of combustion gases.
Table 3. 2 Evaluation tests performed and equipment used.
Property tests Equipment
Dimensional stability and unit density Digital caliper
Bulk density Grain bulk density apparatus
Particle density Gas multipycnometer
Hardness Instron testing machine
Durability Dural tester
Moisture absorption Humidity chamber
Combustion energy Bomb calorimeter
Emissions Grain burning stove
3.3.1 Dimensional stability
The dimensional stability test was performed only on the flax shive pellets
produced using the CPM. Ten randomly selected pellets from each run were
subjected to the dimensional stability test. The length and diameter of the individual
pellets were measured after the pellets cooled and the length/diameter ratio of the
pellets was calculated. Pellets from all 20 runs were sealed in bags and stored at
room temperature for 21 days. Then, the length and diameter measurements were
53
taken again and the changes in the length, diameter and length/diameter ratio of
individual pellets were calculated in terms of percentage.
3.3.2 Densities of pellets
The length and diameter of the pellets produced by the single pellet
equipment were measured just after removal from the die chamber. The unit density
of these pellets (total of 27 single pellets) was calculated based upon the volume and
the mass.
For the pellets formed using the CPM, the dimensions (i.e., length and
diameter) of 10 randomly selected pellets from all 20 runs, total 200 pellets, were
measured to determine the average unit density. Bulk and particle densities and
porosity were also determined using the same equipment and methods as described
previously for the raw material. The bulk density of pellets from all 20 runs was
performed in triplicate, whereas the particle density of pellets (about 10g pellets
samples) was determined without replication. In addition, the porosity of the pellets
was calculated.
3.3.3 Hardness
The hardness of pellets made from the shives from Biolin Research Inc. was
determined by a compression test (ASAE standard S368.2) using the Instron testing
machine. The hardness test assembly is shown in Figure 3.12. Two parallel
horizontal flat plates with a diameter of 57.2 mm were used with a crosshead speed
of 10 mm/min. This is identical to the method which was used by Tabil (1996). The
measurements were performed on ten randomly selected pellets from each of the 20
54
runs. The maximum force in Newtons (N) needed to break the pellets was
determined and was used as a measure of its hardness.
Figure 3. 12 Hardness test assembly.
3.3.4 Durability
Durability of the flax shive pellets from the single pelleter was determined by
a drop resistance test similar to the method used by Sah et al (1980), Khanhari et al
(1989), Shirivastava et al (1989) and Al-Widyan and Al-Jalil (2001). Prior to the test,
the pellets were stored in a refrigerator to ensure that they had the similar moisture
content. They were dropped from a height of 1.85 m onto a steel metal plate surface
three times per each sample. A total of 27 pellets were subjected to the drop
resistance test. The durability of single pellets for each sample was calculated as the
percentage of pellet mass retained. The pellet durability index (PDI) or durability was
calculated using
( ) x100droppingbeforepelletsofMass
droppingafterpelletsofMass%Durability = (3.15)
55
The durability of the pellets produced by the CPM was evaluated by a
different method. In this experiment, the durability of the pellets was determined
using the DURAL tester which was developed at the University of Saskatchewan for
testing alfalfa cubes (Sokhansanj and Crerar, 1999). Before the durability test, the
pellets were sieved using a 5.56 mm round hole sieve to remove any dust. A 100g
sample of pellets was randomly selected, weighed and placed in the tester. The tester
was run for 30 s at a speed of 1600 rpm to tumble the pellets. The sample was then
sieved using the same screen as before the test. The durability of the sample was
expressed as the ratio of the remaining mass of the pellets to the initial mass. Three
replicates of this test were done on pellets from all 20 runs. The pellet durability
index (PDI) or durability was calculated using
( ) 100xtumblingbeforepelletsofMass
tumblingafterpelletsofMass%Durability = (3.16)
3.3.5 Moisture absorption
A sample of about 150g of the pellets produced by the CPM for each of the
20 runs were subjected to a moisture absorption test using a temperature and
humidity conditioner oven (Model AH-213, BRYANT Manufacturing Associates,
Ayer, MA). The moisture content of the pellets was measured and they were then
sealed in plastic bags and kept in the fridge for about a week. The reason to store the
samples in the fridge was that the time between making the pellets and taking the
measurement could vary among treatments. The moisture content of the samples was
determined again after removing them from the fridge. The pellets were placed in the
oven for three days at a temperature of 25°C and humidity of 90%. The mass of
56
pellets from each sample was measured for every 24 h for three days to determine the
percentage change in moisture content of the samples.
3.3.6 Combustion energy
The combustion energy of the individual biomass pellets produced using the
single pelleter and the CPM was measured using the bomb calorimeter discussed
previously. However, for the pellets produced by the CPM, only the pellets made
from flax shives ground with a hammer mill screen size of 4.8 mm were used for
energy content determination. These pellets were made with 18% by weight canola
meal and had a moisture content of around 10% (w.b). The experiment was
performed three times on samples whose mass ranged from 0.5 to 1.0g.
3.4 Emission Measurement
Emission tests of the fuel pellets were performed using the Prairie Fire Multi
Fuel Stove (Prairie Fire Grain Energy, Bruno, SK) shown in Figure 3.13.
Approximately 3kg of pellets containing 22% and 30% by weight canola meal were
used for the emission measurements. For comparison purposes, wood pellets were
also tested.
A small quantity of pellets was also placed in the firebox (Figure 3.14a) to
start the fire. For the burning of the fuel pellets, the feeding rate of fuel pellets was
controlled by setting the fuel control knob at the middle level and room temperature
by setting the temperature control knob at position between low and high level, as
shown in Figure 3.14c and 3.14d. In the beginning, it was necessary to press the
prime button continually for about few minutes to start the feeding auger. Once the
57
temperature inside the stove rose, the automatic fuel feeder started working
normally. After 15-20 minutes of burning pellets, the combustion process was
assumed to be in a steady state and the first gas sample was taken using a needle and
syringe at the outlet shown in Figure 3.14(b). Four samples were taken every 2 min
to evaluate the emissions. This evaluation was conducted using a gas
chromatography (GC) analysis in the Department of Soil Science at the University of
Saskatchewan. Nitrogen (N), oxygen (O2), methane (CH4), nitrogen oxide (NO2),
and carbon dioxide (CO2) were measured and analyzed.
Figure 3. 13 Grain burning stove used to measure emissions.
58
(a) (b)
(c) (d)
Figure 3. 14 Some components of the grain burning stove; (a) Firebox, (b) Outlet where gas samples were collected, (c) Fuel control knob, (d) Room temperature control knob.
3.5 Statistical Analysis
SAS statistical software (SAS Institute, Cary, NC) was used for the analysis
of some of the data that was collected. For the specific heat capacity of the flax
shives, temperature and moisture content were taken as the main independent
variables. Using SAS, the significance of differences among moisture content and
temperature, as well as, the interaction of moisture content and temperature at
different levels was analyzed using a linear regression model.
The effect of grinding on single pellet density and durability was determined
using a completely randomized design. The experimental data analysis was carried
59
out using an analysis of variance (ANOVA) to find differences and significance
among the treatments.
For the pelleting of shives using the CPM, the effects of three variables;
moisture content, particle size of shive particles, and addition of canola meal; and
their interactions with dependant variables (i.e., density, hardness, durability, etc.) of
the densified pellets were analyzed using a SAS General Linear Model (GLM) and
multiple regression models. GLM includes estimation procedures for parameters in
models for a wide range of error distributions. Further investigation to find any
unknown effects that were missed with ANOVA was investigated using a linear
regression with the various methods. For normal distribution of errors ANOVA and
regression methods are used which are based on least squares estimation procedures.
The least square means are estimates of the means from parameters in a least squares
model. They are weighted for the number of replicates in the various treatments. If
all treatments have equal replications, errors are normally distributed and the least
square means are equal to arithmetic means.
60
4. RESULTS AND DISCUSSION
This chapter presents the experimental results of the study in five parts. First,
the physical properties, such as moisture content, particle size and size distribution,
bulk and particle densities and angle of friction of flax shives and canola meal are
presented and discussed. Second, the chemical composition, specific heat capacity
and combustion energy of flax shives are presented. Third, the effect of particle size
(i.e., screen size) on density and durability of the single pellets is presented. Fourth,
the effects of particle size, moisture content and canola meal levels on the various
properties of fuel pellets made from the flax shives obtained from Biolin Research
Inc. are presented and discussed. Lastly, the combustion energy and emission of flax
shive pellets and commercial wood pellets are compared and discussed.
4.1 Physical Properties of Flax Shives and Canola Meal
Table 4.1 indicates which physical properties were determined for the flax
shives from the two sources and the canola meal.
Table 4. 1 Measured physical properties of flax shives and canola meal.
Properties
Source of coarse flax shives
Source of ground shives Canola
meal Biofibre Inc.
Biolin Inc.
Biofibre Inc.
Biolin Inc.
Moisture content yes yes yes yes yes Particle size yes yes yes yes no Size distribution yes yes yes yes no Bulk density yes yes yes yes yes Particle density no no yes yes yes Angle of friction no no no yes no
61
4.1.1 Moisture content
The initial moisture content of the coarse flax shives from both sources was
about 10.5% (w.b.). The average moisture content of flax shives after the three
grinding treatments is given in Table 4.2.
Table 4. 2 Moisture content of flax shives.
Screen size (mm)
Moisture content (% w.b.)*
Biofibre Inc Biolin Inc
Unground 10.5 (0.09) 10.6 (0.02)
6.4 10.4 (0.52) 8.6 (0.08)
4.8 N/Aa 8.2 (0.05)
3.2 10.0 (0.06) 7.9 (0.03)
2.4 9.6 (0.03) N/Aa
*Numbers in parenthesis are standard deviations (n=3). aData not available.
The results show that grinding with a smaller screen size caused a decrease in
the moisture content of the shives. This is due to extra heat generation during
grinding with a smaller screen. Grinds with a large screen had more variations than
those ground with a small screen. The canola meal used as a binding agent in
pelleting of shives had an initial moisture content of 6.63% (w.b.).
4.1.2 Particle size
The initial particle size of the biomass has an effect on the energy
consumption required for grinding (Mani et al. 2004a). Therefore, particles sizes of
the flax shives were determined in this research. For the raw flax shives from
Biofibre Industries Inc., the initial geometric mean chop size was 2.42 mm, while the
62
geometric mean chop size was 8.46 mm for flax shives from Biolin Inc. Geometric
standard deviations of the chopped shives were 3.31 mm and 2.63 mm for Biofibre
and Biolin shives, respectively. The geometric mean particle size (dgw) for ground
shives from both sources, along with the geometric standard deviation (Sgw), is given
in Table 4.3.
Table 4. 3 Geometric mean particle size (dgw) and standard deviation (Sgw) of shives.
Screen size (mm)
Geometric mean particle size (dgw) (mm)*
Biofibre Inc Biolin Inc.
6.4 0.438 (0.012) 0.636 (0.025)
4.8 N/Aa 0.547 (0.020)
3.2 0.408 (0.014) 0.454 (0.009)
2.4 0.367 (0.009) N/Aa
*Numbers in parenthesis are geometric standard deviations of particle size, Sgw (n=3). aData not available.
Grinding of Biofibre shives with a Buhler hammer mill using screen sizes
from 6.4 to 2.4 mm reduced the geometric mean particle size from 0.438 to 0.367
mm, respectively. Grinds from the screen size of 2.4 mm had the lowest standard
deviation of 0.009 mm. For grinding with other screen sizes, differences in the
average particle size were varied slightly. For Biolin shives, the use of hammer mill
screens of 6.4, 4.8, and 3.2 mm resulted in the geometric mean particle size with
values of 0.636, 0.547, and 0.454 mm, respectively. Again, grinding with a hammer
mill with screen sizes from 6.4 to 3.2 mm reduced the geometric mean particle size
from 0.636 to 0.454 mm. The lower standard deviation at a screen size of 3.2 mm
indicates that the material became more uniform in size. The effects of grinding
shives with various screen sizes on particle size and particle size distribution were
analyzed using one-way ANOVA within the SAS statistical software. The ANOVA
63
of particle size of flax shive grinds from the two sources at the three screen sizes
(Table B.1) found that the differences mentioned above were significantly different
(p<0.01). The reduction in the geometric mean particle sizes due to grinding was
slightly different for the shives, even when the same screen size was used. However,
the results are comparable to other studies. Mani et al. (2004a) reported that the
geometric mean particle sizes of corn stover and switchgrass were 0.41 mm at a
moisture content of 6.22% (w.b.), and 0.46 mm at a moisture content of 8.00%
(w.b.), respectively. Both biomass grinds resulted from a hammer mill screen size of
3.2 mm. Shaw and Tabil (2006) reported that the geometric average of particle size
of flax shives was 0.64 mm using a hammer screen of 6.4 mm. Using 6.4 and 3.2 mm
screen sizes, Mani et al. (2004b) determined the geometric mean particle sizes of
corn stover grinds at a moisture content of 7% (w.b), were 0.682 and 0.407 mm,
respectively.
4.1.3 Particle size distribution
Figure 4.1 compares the particle size distribution after grinding of the flax
shives from Biofibre Industries Inc. The most material were retained on the sieve
with an opening of 0.595 mm for all grinding treatments. However, there is a large
difference between the treatments for the sieve with an opening of 1.190 mm.
Overall, the grinds from the various screens had a large size distribution. The reason
for only a small shift in the size distribution was attributed to the removal of large
pieces of flax straw from the raw material before grinding.
64
Figure 4. 1 Particle size distribution of shives from Biofibre Industries Inc. at various screen sizes.
A chart of the particle size distribution of the grinds from the flax shives from
Biolin Research Inc. is shown in Figure 4.2. The use of more sieves resulted in a
better separation of particles. The majority of the particles were retained on the
sieves with openings from 1.190 to 0.595 mm. This may have been due to the fact
that the material was not subjected to an initial screening before grinding. Mani et al.
(2004a) used sieve sizes with openings ranging from 0.09 to 2.00 mm to determine
the particles size distribution of various biomass grinds at various screen sizes. They
found that the grinds from screen size of 3.2 mm had the largest size distribution.
65
Figure 4. 2 Particle size distribution of shives from Biolin Research Inc. at various screen sizes.
4.1.4 Bulk and particle density
The results of the bulk and particle density testing on the ground shives,
including the porosity, are given in Table 4.4. The initial bulk density of shives (i.e.,
raw material) from Biofibre Industries Inc., was determined to be, on average, 133.1
kg/m3.
Table 4. 4 Means of bulk and particle density and porosity of ground flax shives from Biofibre Industries Inc.
Screen size (mm)
Bulk density (kg/m3)*
Particle density (kg/m3)*
Porosity (%)
6.4 156.6 (2.92) 1175 (19.74) 86.67
3.2 162.7 (2.57) 1228 (6.17) 86.75
2.4 166.4 (2.73) 1285 (10.38) 87.50
*Numbers in parenthesis are standard deviations of densities of shives (n=3).
66
Both the bulk and particle densities of ground flax shives increased as the screen
sizes decreased. The porosity of the ground flax shives was essentially the same
amongst the grinding treatments. Therefore, the screen size did not affect the
porosity. When the bulk density of shives ground with the 3.2 mm screen was
compared to other studies, it was found to be higher. This may be due to the care that
was taken to remove the larger pieces of straw, which contain fiber, from the raw
material.
Table B.2 shows that the statistical effects of different screen sizes on the
bulk and particle density of shives from Biofibre Industries Inc. determined using
ANOVA. Use of a smaller screen caused a statistically significant increase in the
both bulk and particle densities of shives. For corn stover grinds, Mani et al. (2004b)
noticed that the particle density of corn stover grinds at a moisture content of 7%
(w.b.) increased from 1085 kg/m3 to 1210 kg/m3 when the hammer mill screen size
decreased from 6.4 to 3.2 mm. Unground shives from Biolin Research Inc. had an
initial bulk density of 71.2 kg/m3. The average bulk and particle densities and the
porosity of the ground flax shives are given in Table 4.5. There were large variations
with the used of screen sizes of 6.4 and 3.2 mm for the particle density of shives.
The bulk and particle densities of the canola meal ground with a screen size of 3.2
mm were 396.3 kg/m3 and 1368.92 kg/m3, respectively.
Table 4. 5 Means of bulk and particle density and porosity of ground flax shives from Biolin Research Inc.
Screen size (mm)
Bulk density (kg/m3)**
Particle density (kg/m3)**
Porosity (%)
6.4 113.3 (6.03) 1312 (28.86) 91.36
4.8 133.7 (6.52) 1314 (9.91) 89.82
3.2 151.3 (5.59) 1338 (42.33) 88.69
*Numbers in parenthesis are standard deviations of ground shives (n=3).
67
ANOVA Table B.3 shows how the different screen sizes used for grinding
shives from Biolin affected the particle and bulk densities. Using ANOVA, it is
shown that use of a smaller screen has a significant effect on the bulk density, but
not on the particle density.
4.1.5 Frictional behaviour of biomass grinds
The results from the determination of the friction and cohesion of flax shives
from Biolin Research Inc. are given in Table 4.6. Regression equations were
estimated for data from the three hammer mill screen sizes at four levels of normal
loads (100, 200, 300, 400N) for the angle/coefficient of internal friction and
cohesion. From Table 4.6, cohesion increased when smaller screen sizes (and
consequently smaller particle size) were used. The use of a small screen resulted in a
decrease in the angle of the internal friction of shives. The coefficient of
determination (R2) was higher. Figure A.1 also shows that there is a linear
relationship between shear stress and normal stress of ground shives.
Table 4. 6 Angle of internal friction and cohesion of shives.
Screen sizes (mm) µ i φi
(Degree)
*Cohesion Cc
Estimate (kPa) R2 SEE
Small (3.2) 0.14 8.01 3.83 0.91 0.54
Medium (4.8) 0.15 8.69 3.81 0.95 0.42
Large (6.4) 0.20 11.23 2.18 0.92 0.69
*Values are averages of three replicates. µ i: coefficient of internal friction. φi: angle of internal friction. SEE: standard error of estimate of cohesion.
68
Mani and co-workers (2004b) found that the adhesion of corn stover grind
increased from 1.85 to 2.79 kPa at a moisture content of 11% (w.b.) as the hammer
mill screen size decreased from 6.4 to 3.2 mm. It was also stated that the coefficient
of wall friction decreased from 0.19 to 0.18 as the hammer mill screen size increased
from 3.2 to 6.4 mm on a polished steel surface.
4.2 Chemical and Thermal Properties
In this section, chemical composition of both flax shives and canola meal is
presented. Also, the specific heat capacity of the shives and combustion energy of
flax shives are presented.
4.2.1 Chemical composition
The chemical compositions of the flax shives and the canola meal used as a
binder are presented in Table 4.7. For comparison, results from other studies are
included in the table.
Table 4. 7 Chemical composition of flax shives and canola meal (percent dry matter basis).
Biomass Protein (%)
Crude fiber (%)
Fat (ether extract)
(%)
Ash (%)
Cellulose(%)
Hemi- cellulose
(%)
Lignin(%)
Flax shives 3.01 65.59 3.11 3.53 53.27 13.62 20.53
Canola meal 30.51 11.40 30.57 6.09 10.40 5.43 6.47
The flax shives, in this research, had a moisture content of 7.9% (w.b.), while
the canola meal had a moisture content of 6.6% (w.b.). The flax shives had a
relatively large amount of crude fiber, cellulose and hemicelluloses that made up the
69
most significant portion in their chemical composition. Some of the components
include a mixture of the other components in the table. Also, they contained a
relatively high amount of lignin that can support the particles during densification.
However, canola meal has a large percentage of protein that helps to bind particles of
shives during the densification process. This also can support densification as a
lubricating agent for the pelleting equipment.
4.2.2 Specific heat capacity of flax shives
The average heat capacity of flax shives ranged from 1.5 to 2.7 kJ/(kg°C)
depending on the temperature and moisture content (Table 4.8). To analyze the
results, a multiple regression analysis with stepwise selection method at a 95%
confidence level was conducted. The variables in this analysis were the first and
second orders of temperature T and T2, moisture content X, and the interaction of
moisture content and temperature XT. The first order of moisture, and interaction
between the first orders of moisture and temperature had the significant effect on the
heat capacity of flax shives. As a result of the analysis, the specific heat capacity (Cp)
was estimated using the regression equation below.
Cp=460.188+120.532X+0.871XT-0.073TX2+0.008XT2 (4.1)
The coefficient of determination (R2) was equal to 0.9575. Figure 4.3 illustrates the
variation of the estimated specific heat capacity of the flax shive particles as a
function of temperature and moisture content. The specific heat capacity of shive
particles increased when both the temperature and moisture content increases.
70
Figure 4. 3 Graph of estimated specific heat capacity of flax shives with respect to moisture content and temperature.
Table 4. 8 Specific heat capacity of flax shives at six levels of temperature and three levels of moisture content.
Temperature (°C)
Specific heat capacity J /(kg °C)
Difference (%)
Measured Estimated*
Moisture content at 8 % (w.b.)
25 1507 1520 0.9
35 1586 1548 0.4
45 1645 1651 0.4
55 1714 1735 1.2
65 1814 1831 1.0
80 1990 1998 0.4
Moisture content at 11 % (w.b.)
25 1884 1857 1.4
35 1942 1915 1.4
45 2008 1989 0.9
55 2091 2080 0.5
65 2197 2188 0.4
80 2389 2382 0.3
Moisture content at 14 % (w.b.)
25 2132 2161 1.3
35 2190 2204 0.6
45 2259 2268 0.4
55 2350 2354 0.2
65 2459 2460 0.0
80 2665 2660 0.2
71
Table 4.8 compares the measured and estimated specific heat capacity values of the
flax shive particles for six levels of temperature and three levels of moisture content.
The difference between the measured and estimated values of the specific heat
capacity was calculated using the following equation.
Difference % = 100 x [Cp measured- Cp estimated]/Cp measured (4.2)
There is a good agreement between the measured and estimated values, i.e., the
maximum differences between the two did not exceed 1.4%.
4.2.3 Combustion energy
The temperature change of water during the combustion of flax shives is
shown in Figure A.2. The average combustion energy of the flax shives was 17.67
MJ/kg at a moisture content of 8.1% (w.b.). This is comparable to the combustion
values of other lignocellulosic materials. Shaw (2008) found that the combustion
values of poplar and wheat straw were 17.76 MJ/kg at the moisture content of 8.2%
(w.b.) and 17.04 MJ/kg at a moisture content of 8.0% (w.b.), respectively. These give
differences of 0.5% and 4.1% from the poplar and wheat straw, respectively.
4.3 Effect of Flax Shive Particle Size on Pellet Density and Durability
The pellets that were made from the flax shives are shown in Figure 4.4. A
single-pelleter was used to manufacture the pellets. One-way ANOVA was used to
analyze to determine the significance of particle size on pellet properties.
72
Figure 4. 4 Flax shive pellets produced by a single-pelleter.
4.3.1 Unit density of pellets made with single-pelleter
Mean unit density of pellets is shown in Table 4.9. The unit density from 6.4,
3.2, and 2.4 mm screen sizes was 1010, 1004, and 1000 kg/m3, respectively. The
variation for unit density was small among replicates within treatments.
Table 4. 9 Unit density and durability of pellets at three levels of grinding.
Screen sizes (mm)
Moisture content (%, w.b.)
Unit density* (kg/m3)
Durability* (%)
2.4 9.6 1010 (3.16) 88.0 (1.13)
3.2 10.0 1004 (2.06) 80.8 (0.56)
6.4 10.4 1000 (1.96) 63.1 (0.56)
*Value in parenthesis is standard error; n = 9.
One-way ANOVA was used to determine the significance of particle size on
the unit density. The ANOVA of unit density of pellets (Table B.4) showed that there
was a significant difference due to the different grinding treatments. A small screen
size resulted in pellets with a higher unit density than those produced with a large
screen.
73
4.3.2 Pellet durability
The mean durability values for the pellets produced using the single-pelleter
are given in Table 4.9. The durability of the pellets increased as the size of the screen
used for grinding decreased. This is likely due to smaller particles filling voids in the
pellets and rearrangement during pelleting.
Once again, one-way ANOVA was used to determine the significance of
particle size. These results are given in Table B.4. The reduction in size and variation
in particle size resulted in pellets that were statistically significant differences in
durability.
4.4 Effect of Particle Size, Moisture Content, and Canola Meal Levels on the
Physical Properties of Flax Shive Pellets Produced in the Pilot-scale Pellet
Mill
ANOVA and multiple regression analysis were used to determine the effects
of particle size, moisture content, and canola meal on dimensional stability, unit
density, bulk and particle density, porosity, hardness, durability, and moisture
absorption of the flax shive pellets. Pellets that were produced using the CPM CL-5
pilot-scale pellet mill are shown in Figure 4.5.
Figure 4. 5 Flax shive pellets produced using a pilot-scale pellet mill.
74
4.4.1 Dimensional stability
The dimensional stability of the pellets was determined in terms of the
change in length (percentage), diameter, and ratio of length to diameter (Table 4.10).
Table B.5 contains the ANOVA results that show how the length, diameter, and ratio
of length to diameter are affected by various factors. The change in length was
greater for a moisture content of 8% than for 14% (w.b.). Pellets formed at higher
moisture content did not absorb much more moisture, and therefore, the pellet length
was not increased as much. However, the pellets formed at a moisture content of
11% (w.b.) had the smallest change in length. The smallest changes in the length of
pellets were those from a treatment containing 18% canola meal using the screen size
of 6.4 mm, while the largest change observed in the length of the pellets from a
treatment containing 21% canola meal using 4.8 mm screen, respectively.
Table 4. 10 Percent change in length, diameter, and ratio of length/diameter of pellets during storage with standard error in parentheses.
Parameters Level n Length (%)
Diameter (%)
Length/Diameter (%)
Canola meal, %
18 5 0.71(0.10) 0.56(0.10) 0.14(0.10)
21 10 1.61(0.09) 0.58(0.9) 1.04(0.09)
24 5 1.30(0.10) 0.66(0.10) 0.65(0.10)
Moisture content, % (w.b.)
8 5 1.39(0.10) 0.46(0.10) 0.93(0.10)
11 10 0.94(0.09) 0.60(0.09) 0.34(0.09)
14 5 1.30(0.10) 0.75(0.10) 0.55(0.10)
Screen size, mm
3.2 5 1.08(0.10) 0.60(0.10) 0.49(0.10)
4.8 10 1.43(0.09) 0.60(0.09) 0.83(0.09)
6.4 5 1.12(0.10) 0.60(0.10) 0.51(0.10)
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Once the factors that were determined to be significantly significant from the
ANOVA analysis multiple regression analyses were performed to illustrate the
important two way interactions on change in length (∆l) of pellets. The model for the
effects of screen size (X1) and canola meal content (X2), and their interaction (X1X2)
is given below.
∆l (%) = -2.165+0.237X1+0.151X2-0.011X1X2 (4.3)
Figure 4.6 shows a plot of how interaction of screen size and canola meal content
affects the change in length of the pellets. The graph shows that while the screen size
has a small effect, the canola meal content affected the change in pellet length the
most.
Figure 4. 6 Change in pellet length at various levels of screen size and canola meal content.
The results of the multiple regression analysis for the interaction of screen
size (X1) and moisture content (X3) are shown in Figure 4.7. The equation for the
model is shown below.
∆l (%) = -2.089+0.692X1+0.281X3-0.062X1X3 (4.4)
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The graph shows that essentially two maximum and two minimum length changes
exist for the factors considered. The largest screen size combined with the lowest
moisture content and the smallest screen size combined with the highest moisture
content resulted in the largest changes in pellet length. The opposite of these, i.e.,
smallest screen size with highest moisture content and largest screen size with lowest
moisture content resulted in the smallest changes in pellet length.
Figure 4. 7 Change in pellet length at various screen sizes and moisture contents.
The results of the regression analysis for the third two-way interaction, i.e., change in
length of the pellets at various levels of canola meal (X2) and moisture content (X3),
is shown below.
∆l (%) = 8.283-0.336X2-0.846X3+0.04X2X3 (4.5)
The results of the interaction of moisture content and canola meal are shown in
Figure 4.8. At the lowest moisture content, the change in pellet length was not
affected by canola meal content. However, both the maximum and minimum length
changes occurred at the highest canola meal content.
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Figure 4. 8 Change in pellet length at various canola meal and moisture content.
The statistical analysis (ANOVA) of the data regarding change in diameter
for the pellets showed that there was a significant effect only for the interaction of
moisture content and canola meal (as shown in Table B.5). However, backward
selection of parameters in linear regression models showed that levels of moisture
content (X3) and canola meal (X2), and the interaction between these factors (X3X2)
were all statistically significant.
∆d (%) = 6.893-0.325X2-0.602X3+0.031X2X3 (4.6)
Figure 4.9 shows the results of the regression analysis of the interaction between
moisture and canola meal contents. The most significant change in diameter occurred
at the highest moisture content when the highest canola meal used. This estimate
agrees with the experimental results in which the largest change in diameter was
1.11% at a moisture content of 14% (w.b.) and canola meal content of 24%. Similar
trends were found by Mani et al. (2006a) in terms of changes in length and diameter
of briquettes produced from corn stover related to the moisture content. Shaw (2008)
78
found that a decrease in the raw feedstock particle size resulted in a decrease in the
lateral expansion.
Figure 4. 9 Lateral expansions of pellets containing three level canola meal produced at three level moisture content.
The effect of the canola meal content on the ratio of length/diameter (∆l/d) of
pellets appeared to be quadratic. Therefore, a second-order parameter for canola meal
content (X2)2 with the first-order parameters, the screen size (X1) parameter, canola
meal content (X2), and a parameter for their interaction (X1X2) was included in the
model.
∆l/d (%) = -12.212+0.539X1+1.008X2-0.019(X2)2-0.025X1X2 (4.7)
In Figure 4.10, the change of the length to diameter ratio of the pellets increased with
increasing the canola meal content. However, the screen size did not significantly
affect the change of this ratio.
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Figure 4. 10 Length to diameter of the flax shive pellets at various screen size and canola meal content.
First order parameters for screen size (X1) and moisture content (X3), their
interaction (X1X3) were used to produce a multiple regression equation 4.8.
∆l/d (%) = -2.091+0.682X1+0.23X3-0.061X1X3 (4.8)
Figure 4.11 illustrates the interaction between screen size and moisture content.
Figure 4. 11 Change in length to diameter ratio of the flax shives pellets at various screen sizes and moisture contents.
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The trend shown in Figure 4.11 is similar to that of Figure 4.7 which illustrates the
effect of screen size and moisture content on the change in pellet length. This is as
expected since the change in diameter was not significantly affected by either of
these factors. This leaves the change in the ratio dependent only on the length
change.
4.4.2 Unit density of pellets
Table B.6 shows the results of an ANOVA of unit density of pellets with
regard to screen size, canola meal content and moisture content. This shows that
there are no significant effects from any of the factors considered or their interactions
on the unit density. Least squares means of unit density values of the pellets that
were formed using the parameters listed are given in Table 4.11.
Table 4. 11 Least squares means of unit density of shive pellets.
Parameters Level n Unit density
(kg/m3) Standard error
Screen size, mm 3.2 5 1134 28.2
4.8 10 1160 25.7
6.4 5 1140 28.2
Moisture content, % (w.b.) 8 5 1186 28.2
11 10 1140 25.7
14 5 1109 28.2
Canola meal, % 18 5 1188 28.2
21 10 1151 25.7
24 5 1095 28.2
After the ANOVA failed to identify factors that may have effects on unit density, a
linear regression was performed and the following model was selected.
Unit density = 986.408+14.225X2+44.108X2-2.708X2X3 (4.9)
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Figure 4.12 shows the effect of the interaction between canola meal (X2) and
moisture contents (X3) with regard to the unit density of the pellets. There appeared
to be a difference in the unit density between high and low moisture content when
high levels of canola meal were used. This illustrates that the advantage of high
levels of canola meal is reduced for making pellets with high unit density when the
flax shive mixture had high enough moisture. However, moisture content did not
have a large effect on the unit density at low canola meal.
Figure 4. 12 Unit density of the pellets produced at various levels of moisture and canola meal contents.
This result is in agreement with other studies. Mani et al. (2006a) found that the
density of corn stover briquettes produced with 5 to 10% (w.b.) moisture content was
higher than those produced at a moisture content of 15% (w.b). In addition,
Wamukonya and Jenkins (1995) reported that high quality briquettes were made
from sawdust and wheat straw when produced at a moisture content ranging from 12
to 20% (w.b.).
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4.4.3 Bulk and particle density
Table B.7 lists the results of an ANOVA performed on the data for the bulk
and particle densities and porosity. For the bulk density, the analysis did not indicate
any statistically significant effects with regard to screen size, canola meal content,
moisture content or interactions among these factors. Table 4.12 shows the least
squares means of bulk density, particle density, and porosity along with the standard
errors.
Table 4. 12 Bulk and particle density and porosity of flax shive pellets.
Parameters Level Bulk density* (kg/m3)
Particle density* (kg/m3)
Porosity* (%)
Screen size, mm 3.2 653.7±23.7 1192±9.9 43.4±1.7
4.8 667.9±21.7 1144±9.0 39.5±1.6
6.4 662.3±23.7 1118±9.9 38.5±1.7
Moisture content, % (w.b.) 8 682.3±23.7 930±9.9 25.9±1.7
11 652.8±21.7 1314±9.0 50.4±1.6
14 648.8±23.7 1210±9.9 45.1±1.7
Canola meal, % 18 686.6±23.7 1147±9.9 37.5±1.7
21 665.2±21.7 1152±9.0 40.0±1.6
24 632.0±23.7 1155±9.9 43.9±1.7
*Mean±Standard error.
To further investigate the effects of the factors on the pellet bulk density, a
linear regression analysis with backward selection method was performed. The
regression model including parameters for moisture (X3) and canola meal contents
(X2), and their interaction (X2X3) is shown below.
Bulk density = 901.181+2.976X2-29.057X3-2.114(X3)2-1.096X2X3 (4.10)
The second order of moisture content (X3)2 was also considered in the model. Figure
4.13 shows that the highest bulk density resulted from the smallest amount of canola
meal and lowest moisture content.
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Figure 4. 13 Bulk density of flax shive pellets at various moisture and canola meal content.
The regression model using screen size (X1), canola meal content (X2), and
their interaction (X1X2) as parameters is shown below.
Bulk density = 1006.14-33.195X1-17.287X2+1.709X1X2 (4.11)
Figure 4.14 shows the results of these factors have on the bulk density of pellets. The
bulk density increased as the canola meal content decreased. However, the effect of
the canola meal content was more significant for the 3.2 mm screen.
Figure 4. 14 Bulk density of flax shive pellets at various levels of screen size and canola meal.
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The final regression model created for the bulk density considered screen size (X1),
moisture content (X3), and their interaction (X1X3). It included a second order term
of moisture content (X3)2.
Bulk density = 957.629+1.259X1-52.707X3+0.13X1X3+2.114(X3)2 (4.12)
Figure 4.15 shows the effect of screen size and moisture content on bulk density.
There was a significant effect of the moisture content on the bulk density of the
pellets. The maximum bulk density occurs at a moisture content of 8% (w.b.) and the
minimum is between 11 and 14% (w.b.) moisture. The effect of screen size was
insignificant.
Figure 4. 15 Effect of screen size and moisture content of bulk density of pellets.
The ANOVA results for particle density, shown in Table B7, indicate that all
the main effects and interactions that were considered are statistically significant.
The least squares means of the particle density were calculated for each variation of
screen size, moisture content and canola meal content. The results are shown in
Table 4.12. The moisture content values suggest that this effect is quadratic as the
middle levels gave the most compact pellets. The regression models for the three
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two-way interactions were used. The model for the effects of screen size (X1), canola
meal content (X2), and their interaction (X1X2) as parameters on the particle density
is given below.
Particle density = 657.603+104.478X1+30.571X2-6.073X1X2 (4.13)
Figures 4.16, 4.17, and 4.18 illustrate the results three two way interactions. Figure
4.16 shows that the particle density of the pellets was higher for larger canola content
and smaller screen size. This is, likely, a result of the higher particle density for the
canola meal. However, the lowest particle density was obtained with the use of larger
screen at higher canola meal content.
Figure 4. 16 Particle density of pellets at levels of screen size and canola meal.
The regression model created for the particle density considered the first
orders of screen size (X1), moisture content (X3), their interaction (X1X3), and the
second order of moisture contents (X3)2.
Particle density = -2166.458-47.641X1+615.848X3+2.236X1X3-26.361(X3)2
(4.14)
The quadratic nature of the regression model with respect to moisture content is
evident in Figure 4.17. The screen size did not have a significant effect on the
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particle density at each level of moisture. However, the moisture content had a large
effect on the particle density.
Figure 4. 17 Particle density of pellets at three levels of screen size and moisture
content.
The regression model was created including parameters for canola meal
content (X2) and moisture content (X3), and their interaction (X2X3) along with the
second order of moisture content (X3)2.
Particle density = -3756.227+64.814X2-5.763X2X3+747.601X3-26.361(X3)2
(4.15)
Once again, the quadratic nature is evident in Figure 4.18. While there are small
differences in the particle density at different canola contents at each moisture level,
the largest change is seen as the moisture level varies.
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Figure 4. 18 Particle density of pellets at three levels of canola meal and moisture
content.
The results of the ANOVA of the porosity data, shown in Table B.7, indicate
that the moisture content had the only significant effect. The least squares means of
porosity given in Table 4.12 also confirm this. Porosity decreased slightly as larger
screen was used and as canola content increased, but their effects were not
significant. Once again, the moisture content showed that the effect on porosity has a
quadratic distribution (Table 4.12).
4.4.4 Hardness
Hardness is another measure of quality of the fuel pellets. This essentially
measures the maximum crushing load of pellets to resist the forces generated during
pelleting, cooling, handling, storage, and transportation. Colley et al. (2006)
measured the hardness of switchgrass pellets as a measure of quality of fuel pellets.
In addition, switchgrass pellet hardness was similar to alfalfa and wood pellets and
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increasing the fuel pellet hardness resulted in a reduction of the dust and fines
generated during their transportation (Jannasch et al. 2001). As for the other
properties, the hardness data was analyzed using ANOVA. This is shown in Table
B.8. The results indicate there is only a significant effect on pellet hardness due to
the level of canola meal. The least squares means given in Table 4.13 are in
agreement with this result.
Table 4. 13 Least squares means and standard error of hardness of shive pellets.
Parameters Level n Hardness (N) Standard error
Screen size, mm 3.2 5 511.3 37.9
4.8 10 482.8 34.6
6.4 5 470.4 37.9
Moisture content, % (w.b.) 8 5 489.6 37.9
11 10 445.2 34.6
14 5 529.7 37.9
Canola meal, % 18 5 616.1 37.9
21 10 448.3 34.6
24 5 400.1 37.9
The result of the regression analysis for the effect of the first order of canola
meal (X2) moisture content (X3), and the second order of moisture content (X3)2 on
the hardness is shown below.
Hardness = 1879.226-35.986X2-128.325X3+6.136(X3)2 (4.16)
Figure 4.19 shows the effect of canola meal and moisture content. The canola meal
content affects the hardness more than the moisture level.
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Figure 4. 19 Effect of canola meal and moisture content on pellet hardness.
4.4.5 Durability
In general, pellets with higher durability are more able to resist forces during
transportation and storage. The results of analyzing the durability of the pellets using
ANOVA are shown in Table B.9. This shows that there is a significant effect due to
moisture content only. The least squares means presented in Table 4.14 show that
there may also be a significant effect due to canola meal content.
Table 4. 14 Least squares means and standard error of durability of shive pellets.
Parameters Level n Durability (%) Standard error
Screen size, mm 3.2 5 70.2 5.5
4.8 10 65.8 5.0
6.4 5 73.7 5.5
Moisture content, % (w.b.) 8 5 62.2 5.5
11 10 65.5 5.0
14 5 82.1 5.5
Canola meal, % 18 5 81.5 5.5
21 10 64.6 5.0
24 5 63.7 5.5
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To more conclusively determine what affected the durability most
significantly, a backward elimination regression was performed. This model suggests
that there is a second-order effect of canola meal content (X2)2 and possibly an
interaction between levels of canola meal and moisture (X2X3).
Durability = 719.280-61.582X2-1.353(X2)2+0.163X2X3 (4.17)
The results are shown in Figure 4.20. The highest durability occurred for the lowest
canola meal content and highest moisture content. The durability was lowest at the
middle canola meal content and least moisture content.
Figure 4. 20 Effect of moisture and canola meal content on pellet durability.
4.4.6 Moisture absorption
All pellets had volume changes due to moisture absorption. The results of the
ANOVA of moisture absorption, shown in Table B.10, and determination of least
squares means, shown in Table 4.15, indicate that the moisture content was the only
factor that had a significant effect.
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Table 4. 15 Least squares means and standard error of moisture absorption of shive pellets.
Parameters Level n Moisture
absorption (% d.b.)
Standard error
Screen size, mm 3.2 5 76.7 5.2
4.8 10 70.2 4.8
6.4 5 73.9 5.2
Moisture content, % (w.b.) 8 5 122.1 5.2
11 10 73.8 4.8
14 5 24.9 5.2
Canola meal, % 18 5 71.3 5.2
21 10 79.8 4.8
24 5 69.7 5.2
The model for the effect of screen size (X1) and moisture content (X2) on moisture
absorption of flax shive pellets is shown the following equation.
Moisture absorption = 254.277-0.864X1-16.207X3 (4.18)
Figure 4.21 shows that the lowest moisture absorption subjected to the pellets that
produced at moisture content of 8% (w.b).
Figure 4. 21 Effect of screen size and moisture content on moisture absorption of
flax shive pellets.
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4.5 Combustion Energy and Emissions
4.5.1 Combustion energy
The combustion energy was measured in triplicate for flax shive pellets that
were produced by both the single-pelleter and pilot-scale pellet mill. Pellets produced
by the single-pelleter had combustion energy of 19.17 MJ/kg while the pellets
formed by the pilot-scale pellet mill had combustion energy 20.47 MJ/kg. Pellets
formed using the pilot-scale pellet mill had a higher combustion energy value than
pellets formed in the single pelleter. The reason for that is due to the extra energy
contained in the canola meal fat in the pellets produced using the pilot-scale pellet
mill. The energy contents of the fuel pellets made from shives were identical to
energy values of the biomass materials. The combustion energy contents were found
for alfalfa stems (18.67 MJ/kg), wheat straw (17.97 MJ/kg), rice straw (15.09
MJ/kg), switchgrass (18.06 MJ/kg), and sugarcane (18.99 MJ/kg) (Jenkins et al.
1998). They also reported that coal had the combustion energy values greater than
those for biomass materials due to their lower degree of oxidation.
4.5.2 Emissions
Emission measurements were performed on flax shive pellets, containing 22
and 30% canola meal, and commercial wood pellets. The flax shives were ground
with the hammer mill screen and the initial moisture content of the mixture was 11%
(w.b.). ANOVA (Table B.11) of emissions of wood pellets and flax shives pellets
with an inclusion of canola meal shows that there is a significant difference. Table
4.16 shows the effects of the two levels of canola meal at the final moisture content
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of 6.9 and 8.3% (w.b.) on various emissions including carbon dioxide, nitrogen,
methane, nitrogen dioxide, and oxygen. Also, the emissions of the commercial
pellets are presented. Olsson and Kjallstrand (2004) reported that emissions of
methane (8 ppm) from burning of softwood pellets in a stove. This value was higher
than the emissions from methane from combustion of shive pellets. Therefore, it can
be concluded that the emissions from burning of fuel pellets from shives will be
negligible.
Table 4. 16 Emissions from flax shive pellets and wood pellets.
Product Canola meal (%)
Emissions*
Nitrogen
(ppt) Oxygen
(ppt) Methane
(ppm) Nitrogen
oxide (ppm)
Carbon dioxide
(ppt)
Shive pellets 22 797.1(0.59) 163.6(2.0) 1.26(0.07) 1.14(0.13) 46.3(1.8)
Shive pellets 30 794.1(0.59) 164.3(2.0) 1.29(0.07) 1.45(0.13) 45.3(1.8)
Wood pellets 0 788.9(0.59) 176.6(2.0) 1.63(0.07) 0.34(0.13) 36.9(1.8)
*Value in parenthesis is standard error, n=4. ppm = parts per million. ppt = parts per thousand.
Pellets made from the two levels of canola meal were not different for any of the
emission measurements. However, they were all different from the emission
measurements taken on the wood pellets.
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4.6 Summary
Grinding of flax shives from both sources with use of different hammer mill
screen sizes gave a significant difference in moisture content and particle size. The
moisture content ranged from 10.50% (w.b.) for unground shives to 9.55% (w.b.) for
finely ground shives (Biofibre), while the moisture content of the Biolin shives
ranged from 10.59% (w.b.) for unground material to 7.89% (w.b.) for shives ground
with a screen size of 3.2 mm. A small hammer mill screen size of 2.4 mm reduced
the geometric mean particle size up to 0.367 mm for Biofibre shives, while the
screen size of 3.2 mm reduced the geometric mean particle size up to 0.454 mm for
the Biolin shives. Also, the use of small screen sizes resulted in a slight shift in the
particle size distribution for ground shives from both sources. The use of a small
screen size resulted in an increase both in the bulk and particle density of shives.
After grinding with a small screen size (3.2 mm), bulk density increased 2.1 times
from the initial bulk density of shives for the Biolin material. The adhesion
coefficient of shives increased from 0.14 to 0.20 as screen size decreased, while the
coefficient of internal friction decreased from 3.83 to 2.18 kPa with the use of
smaller screens.
The flax shives contained a relatively large amount of crude fiber (65.59%),
cellulose (53.27%), hemicelluloses (13.62%), and lignin (20.53%) in their chemical
composition. The canola meal had a large percentage of proteins (30.51%) and fat
(30.57) in its chemical composition. Specific heat capacity of the flax shives ranged
from 1506 to 2664 J/(kg°C) and increased with increasing a moisture content (8-
14%) and temperature (15-80°C). The combustion energy of shives was 17.67 MJ/kg
at a moisture content of 8.1% (w.b.).
95
There was a significant effect of grinding on both unit density and durability
of the pellets. The smallest screen size (2.4 mm) gave the highest unit density (1010
kg/m3) in the pellets. Durability of pellets ranged from 66.0 to 88.0%, and the highest
durability was found in shives ground with a screen size of 2.4 mm. The use of the
smallest screen size resulted in the highest unit density and durability of pellets
produced using the single-pelleter.
The effects of three levels of particle size, moisture content, and canola meal
content on various pellet properties were investigated for pellets made from flax
shives obtained from Biolin Inc. and processed using the pilot-scale pellet mill. At
the lowest moisture content (8%), the change in pellet length increased as screen size
was increased from 3.2 to 6.4 mm. However, at the highest moisture content (14%),
the change was decreased as the screen sizes were increased. There were also the
effects of interaction of screen size and canola meal, and interaction of screens and
moisture content on the change of length of pellets during storage. ANOVA of
change in diameter illustrated only the effect of interaction of moisture and canola
meal content. However, the backward selection of parameters in linear regression
models showed there was a significant effect of moisture and canola meal and their
interaction on change in diameter. At the highest canola meal content, the smallest
change in diameter occurred at the lowest moisture content, while the largest change
in diameter occurred at the highest moisture content. The use of lower canola meal
resulted in the lowest value at a lower moisture content in dimensional stability.
Although there were no significant effects of factors and their interactions on
the pellet unit density from ANOVA, further investigation using a linear regression
with the backward selection method showed the effect of canola meal and moisture
content interaction. The unit density increased with decreasing moisture content at a
96
higher canola meal inclusion. However, moisture content did not have an effect on
the unit density at low canola meal. Unit density was the lowest at the highest canola
meal inclusion and moisture content.
In the regression model using parameters for moisture content, canola meal,
and their interaction, the significant effects on the bulk density of the pellets were
found. The highest bulk density was obtained (682 kg/m3) from shive mixtures using
the smallest amount of canola meal (18%) at the lowest moisture content (8%). At a
higher canola meal content, the bulk density increased by increasing the screen size.
However, there was no effect of the screen sizes at the low canola meal inclusion. A
higher bulk density was obtained from shives with a medium moisture content (11%
w.b.) at a smaller screen size (3.2 mm). The highest particle density was obtained by
using a larger canola meal at a smaller screen size, while the lowest particle density
was obtained with the use of a larger screen at a higher canola meal conclusion. Also,
the highest particle density was obtained when a medium moisture content, smaller
screen and higher canola meal was used.
A higher hardness and higher durability were found for the shive pellets that
were produced with the use of a small amount of canola meal at a higher moisture
level. The moisture absorption of shive pellets was significantly affected by only the
moisture content. However, there were no effects of screen size and canola meal
levels on moisture absorption. Pellets that were produced at lower moisture content
resulted in a higher percentage of moisture. In terms of hardness and durability, the
best level of the processing parameters are a higher moisture content (14% w.b.) and
lower canola meal inclusion (18%) to produce the hardest and more durable pellets.
The combustion energy was 19.17 MJ/kg for single pellets without canola
meal, while the combustion energy of the pellets with canola meal inclusion
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produced by the pilot scale mill was 20.47 MJ/kg. Due to the inclusion of the canola
meal in shives, combustion energy of the pellets was increased. Two levels of canola
meal for shive pellets did not have a significant effect on the emissions. However,
there were significant differences between shive pellets and commercial wood pellets
on the emission measurements. Low emissions of methane (1.29 ppm) and oxygen
(164.3 ppt) were found from the combustion of flax shive pellets. These emissions
were relatively lower than methane (1.63 ppm) and oxygen (176.6 ppt) from the
combustion of commercial wood pellets. The other emissions including nitrogen,
nitrogen oxide, and carbon dioxide were higher for shive pellet combustion than
those emissions from wood pellets.
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5. CONCLUSION
Society has started to use renewable sources of energy as a substitute for non-
renewable fossil fuels as an energy source. Biofuels from biomass can be an
alternative energy source and meet human energy needs. Biomass materials such as
sawdust, shavings, corn stover, switchgrass, and others have been studied
extensively. The major problems are that they are difficult to handle and are costly to
transport and store due to their low bulk density. It is necessary to densify these
materials into pellets or briquettes.
Densification of various biomass is affected by both raw material properties
and process variables. Raw material properties such as particle size, particle size
distribution, densities, moisture content, and chemical components were studied on
various types of biomass before processing. Therefore, the properties of densified
products made from biomass were studied extensively, considering parameters that
included density, hardness, durability, and moisture absorption.
In this research, flax shives from two flax straw processing companies were
used to manufacture biofuel pellets using both a single-pelleting unit and a pilot-
scale pellet mill.
The first objective was to characterize the flax shives from the two sources
and the following conclusions can be drawn.
1. Small screen size (i.e., 2.4 mm screen for the first source and 3.2 mm
screen for the second source) to grind the flax shives resulted in a decrease in
moisture content.
99
2. Shives ground with a screen size (i.e., 2.4 mm) produced a smaller mean
geometric particle size than those ground with a larger screen (i.e.,6.4 mm).
3. Better separation of shive particles to determine the particle size
distribution was achieved by using a larger number of sieves in the separation
process.
4. The use of the smaller hammer mill screen resulted in an increase in both
bulk and particle density of shives.
5. The use of a 3.2 mm screen size as compared to a screen of 6.4 mm
resulted in an increase in the adhesion coefficient of shives from 0.14 to 0.20 and a
decrease in a coefficient of internal friction from 3.83 to 2.18 kPa.
6. The shives contained crude fiber (65.59%), cellulose (53.27%),
hemicelluloses (13.62%), and lignin (20.53%) when the moisture content of the
shives was 7.9% (w.b.).
7. Specific heat capacity of flax shives ranged from 1.5 to 2.7 kJ/(kg°C) and
increased when the moisture content was increased from 8 to 14% (w.b.) and the
temperature was increased from 15 to 80°C.
8. The flax shives had the combustion energy of 17.67 MJ/kg at a moisture
content of 8.1% (w.b.).
The second objective of this study was to find the effect of variables such as
screen size, moisture content, and the addition of canola meal used as a binder on the
properties of the pellets. The following conclusions can be drawn:
1. In the pelleting of flax shives using the single-pelleter, the smallest screen
size (2.4 mm) resulted in the higher unit density (1010 kg/m3) and higher durability
(88%) of the pellets than those produced with the larger screen.
100
2. Pellets manufactured by the pilot-scale pellet mill using the addition of
18% canola meal and the screen size of 6.4 mm at all moisture levels produced the
highest unit density. The highest bulk density (682 kg/m3) was obtained from shive
mixtures with 18% canola at a moisture content of 8% (w.b). Shives with a moisture
content of 11% (w.b.) from a screen size with an opening of 3.2 mm and the addition
of 24% canola meal produced the pellets with highest particle density.
3. In terms of dimensional stability, the lowest change occurred in the length
of the pellets that were produced using 18% canola meal and a moisture content of
14% (w.b). However, the lowest change occurred in the diameter of the pellets that
were made at 8% (w.b) moisture content with mixture containing the highest canola
meal (24%). The lowest change noticed in the ratio of length/diameter of pellets were
those produced at 8% (w.b) moisture content and 18% canola meal from shives
ground with the smallest screen size (3.2 mm).
4. The unit density of pellets increased with decreasing moisture content at
higher canola meal content.
5. The highest bulk density of pellets was obtained from a shive mixture at a
moisture content of 8% (w.b.) with the addition of 18% canola meal.
6. The highest particle density was found in the pellets that were produced
with the use of highest canola meal content (24%) and the smallest screen size (3.2
mm).
7. The only significant effect on the hardness of pellets was the level of
canola meal. The highest hardness was found in the pellets that produced with the
use of 18% canola meal.
8. Since the handling of pellets is very important, durability can be used as a
basis of judging for the overall pellet quality. The highest durability was obtained for
101
the pellets that were produced with less canola meal (18%) at the highest moisture
content (14% w.b.).
9. The only significant effect on moisture absorption of pellets was for
moisture content.The lowest moisture absorption was found for the pellets that were
produced at a moisture content of 14% (w.b.).
The third specific objective of this study was to measure the heat of
combustion of the flax shive pellets and to analyze the gas emissions from burning
pellets. The following conclusions can be drawn:
1. Values of the combustion energy of the flax shive pellets produced with
and without canola meal were 20.47 and 19.17 MJ/kg. The addition of the canola
meal in the shive mixture resulted in an increase in the combustion energy of the
pellets due to the fat content in the canola meal.
2. The canola meal contents (22 and 30%) in the shive pellets did not result in
the difference in emissions. However, significant differences were found between
shive pellets and commercial wood pellets in emissions. Emissions from flax shive
pellets were lower in methane (1.29 ppm) and oxygen (164.3 ppt) than emissions
from commercial wood pellets having methane (1.63 ppm) and oxygen (176.6 ppt).
In conclusion, densification of flax shives into fuel pellets improved the
handling characteristics, increased bulk density and energy content. The most
durable pellets were produced at a higher moisture and lower canola meal content.
Emissions from combustion of densified fuel pellets from flax shives had less
methane and oxygen than emissions from commercial wood pellets.
102
6. RECOMMENDATIONS FOR FUTURE STUDIES
The following recommendations are suggested for future research studies:
1. We need to know more about the costs of grinding and specific energy
consumption. This study indicates that the use of a 3.2 mm screen in a hammer mill
is more than adequate. Grinding of biomass with smaller screen sizes is probably not
required and would be too costly.
2. Moisture content of about 14% (w.b.) probably would be ideal for
densification. Materials that are stored in dry conditions may arrive at the pelleting
plant with a moisture content as low as 8% (w.b.) or lower and in this case a small
amount of water would need to be added. Wet materials with a moisture content of
18% or more would have to be dried. The optimum moisture content for pelleting
needs to be established in more detail.
3. Lower grades of canola meal could be used as a biological binding agent.
The levels of canola meal used in this study were probably higher than the optimum
value. Therefore, more research is needed on the use of lower levels of canola meal
as a binder. Other binders should be considered.
4. There is a need to find out more about the optimum preheating temperature
of materials going into the pelleting equipment. Steam conditioning may help with
the adhesion of particles. Also, the combination of preheating and steam conditioning
of materials may reveal an interaction between these two factors on the quality of
final products.
103
5. Development of equipment that will give commercial levels of throughput
of the pellets is needed. Consideration the processing parameters such as pressure,
time, feedstock speed, and die geometry as related to throughput is necessary.
104
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APPENDIX A
FIGURES SHOWING CHARACTERISTICS OF FLAX SHIVES
113
Figure A.1 Normal stress-shear stress plot for friction measurement of shives.
Figure A.2 Temperature change during combustion of flax shives.
114
APPENDIX B
ANOVA TABLES
115
Table B. 1 Analysis of variance of particle diameter of ground flax shives.
Source df Mean square
Biofibre Inc Biolin Inc.
Screen sizes 2 0.00381478** 0.02489591**
Error 6 0.00014767 0.00038381
**p< 0.01.
Table B. 2 Analysis of variance of densities of ground flax shives (Biofibre Inc.).
Source df Mean square
Bulk density Particle density
Screen sizes 2 73.31* 9168.75**
Error 6 7.53 178.61
**p < 0.01, *p<0.05.
Table B.3 Analysis of variance of densities of ground flax shives (Biolin Inc.).
Source df Mean square
Bulk density Particle density
Grinding treatment 2 1084.96** 629.067ns
Error 6 36.69 907.473
**p < 0.01, ns = not significant.
Table B.4 Mean squares from Analysis of variance of unit density and durability of pellets produced from flax shives obtained from Biofibre Inc.
Source df Unit density Durability
Between groups 2 232.051* 1482.32**
Within groups 24 54.130 5.71
**p < 0.01, * p<0.05.
116
Table B.5 Mean squares from Analysis of variance of change in length, diameter, and ratio of length/diameter of pellets.
Source df Length Diameter Length/Diameter
Screen size (X1) 2 0.011ns 0.001ns 0.004ns
Canola meal (X2) 2 0.548* 0.014ns 0.464*
Moisture content (X3) 2 1.496** 0.104ns 1.675**
X1*X2 4 0.548* 0.028ns 0.504*
X1*X3 2 1.690** 0.046ns 1.250**
X2*X3 1 1.015** 0.622* 0.053ns
X1*X2*X3 1 0.001ns 0.001ns 0.005ns
Error 5 0.051 0.051 0.054
**p< 0.01, *p<0.05, ns=not significant.
Table B.6 Mean squares from Analysis of variance of pellet unit density.
Source df Mean square
Screen size (X1) 2 595.65ns
Canola meal (X2) 2 10928.475ns
Moisture content (X3) 2 8120.461ns
X1*X2 4 159.482ns
X1*X3 2 2180.175ns
X2*X3 1 4753.125ns
X1*X2*X3 1 1596.125ns
Error 5 3982.70
ns=not significant.
Table B.7 Mean squres from Analysis of variance of bulk and particle density, and porosity of shive pellets.
Source df Bulk density Particle density
Porosity
Screen size (X1) 2 130.854ns 18109.112* 74.111ns
Canola meal (X2) 2 3784.314ns 4826.477* 70.673ns
Moisture content (X3) 2 2438.121ns 251507.553* 1087.459*
X1*X2 4 826.749ns 6644.965* 45.014ns
X1*X3 2 129.306ns 30424.375* 83.301ns
X2*X3 1 779.078ns 21520.967* 49.900ns
X1*X2*X3 1 109.216ns 3251.123* 28.125ns
Error 5 2813.389 486.902 15.270
*p<0.01, ns=not significant.
117
Table B.8 Analysis of variance of hardness of flax shive pellets.
Source df Mean square
Screen size (X1) 2 2864.888ns
Canola meal (X2) 2 65009.074*
Moisture content (X3) 2 5842.130ns
X1*X2 4 2386.603ns
X1*X3 2 381.035ns
X2*X3 1 215.541ns
X1*X2*X3 1 2294.930ns
Error 5 7173.317
*p<0.05, ns=not significant.
Table B.9 Analysis of variance of durability of flax shive pellets.
Source df Mean square
Screen size (X1) 2 317.421ns
Canola meal (X2) 2 516.764ns
Moisture content (X3) 2 529.290*
X1*X2 4 63.724ns
X1*X3 2 82.458ns
X2*X3 1 132.641ns
X1*X2*X3 1 12.007ns
Error 5 148.978
*p<0.05, ns=not significant. Table B.10 Analysis of variance of moisture absorption of flax shive pellets.
Source df Mean square
Screen size (X1) 2 128.440ns
Canola meal (X2) 2 185.422ns
Moisture content (X3) 2 11833.223*
X1*X2 4 351.082ns
X1*X3 2 114.605ns
X2*X3 1 0.068ns
X1*X2*X3 1 228.413ns
Error 5 136.852
*p<0.05, ns=not significant
118
Table B.11 Mean squares from Analysis of variance of emissions of flax shive pellets and wood pellets from combustion.
Source df Nitrogen
(ppt) Oxygen
(ppt) Methane
(ppm)
Nitrogen oxide (ppm)
Carbon dioxide
(ppt)
Pellets 2 36.08** 214.7** 0.172** 1.306** 106.8**
Error 9 1.4 15.9 0.019 0.071 12.9
**p<0.01.